mail_daemon: warn about unused return value

GCC 13 warns that using the `begin()` method on a container is marked as
`[[nodiscard]]`. This makes sure that this warning is not treated as an error.

The code actually looks like it does not do much if anything of use. As a
follow-up ticket #18478 is raised.

Change-Id: Ie149f7e02cacda2bcd0be59617583605d5905747
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6655
Reviewed-by: Niels Sascha Reedijk <niels.reedijk@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

IMAP: sync fixes

This patch should fix:

- deadlocks while manual syncing/shutdown;
- crashes while manual syncing/shutdown.

Change-Id: I0962ff5580c19cbf740d002b6f62681ed5f558ab
Reviewed-on: https://review.haiku-os.org/758
Reviewed-by: Stephan Aßmus <superstippi@gmx.de>

mail_daemon: IMAP delete messages on the server only when delete_remote_when_local setting is set

Change-Id: I88fcc7a185cede080af7c3b85f9466a02f895809
Reviewed-on: https://review.haiku-os.org/674
Reviewed-by: Stephan Aßmus <superstippi@gmx.de>

IMAP: Retrieve on-disk folder state async.

* Messages that we don't have info for during synchronization are now
collected, and synchronized once the folder state is available,
instead of waiting for that particular entry to become available
(without proper means to wait).

IMAP: Extra local messages are now deleted.

* When a message on the server is deleted, it will now be deleted
locally, too.

IMAP: setting flags, and body fetching works now.

* The direct methods in BMailProtocol now forward the request to the
looper; it's no longer the mail_daemon's responsibility to know
anything about that protocol.
* It's in desperate need of refactoring, but it doesn't hurt to add
it to the repository as is.

IMAP,POP3,NewMailNotification: fixed x86_64 build.

* Notifier::HeaderFetched(): fixed the method signature (go figure how it could
build on x86).
* POP3: fSizes type is now std::vector<size_t> instead of BList. Please review.
off_t might be a better choice.
* added the mail_daemon add-ons to the Haiku package again.

IMAP: downloading mails is now working.

* Changed the way the attributes are written to make sure that everything
that can be written once is in fact written just once.
* The rename code in BMailProtocol::_ProcessFetchedHeader() was broken,
and caused the hang of the last commit.

Mail Kit: Changed how filter changes work.

* BMailFilter::HeaderFetched() now only alters the entry_ref, and returns
B_MOVE_MAIL_ACTION to move a mail.
* Instead of potentially moving the file around several times, the
BMailProtocol now takes care of carrying out the filter action just once,
including trying to make the file name unique.
* This also allows the IMAP add-on to know the final location of the mail,
and thus downloading a message actually works.
* However, with my test inbox, it currently hangs, and a current Debugger does
not work on my older system -- I guess I need to update.
* Replaced the duplicated space mechanism within the "HaikuMailFormatFilter"
that is substantially faster, and handles all whitespace, not just spaces.
It will also replace tabs with spaces.

IMAP: work in progress of downloading the mail body.

* Most things are in place now, we just try to download the body to the
wrong file, as the final location is currently unknown.
* Added local only kPartialMessage flag for mails, but it's not being
used yet.

IMAP: call the folder's handler like the mailbox.

IMAP: IMAPFolder now stores the last UID.

* So mails are no longer downloaded every time.

imap: Fetching headers is now working.

* No state is stored yet, though, so that all messages will be downloaded
over and over again.

imap: WIP of getting fetching headers to work.

* Changed the FetchListener mechanism quite a bit.
* Doesn't seem to work correctly yet, although most pieces are in
place now.

imap: Work in progress of connection setup.

* SelectCommand now correctly encodes the mailbox name again.
* Hierarchy separators from the LIST/LSUB commands are now properly parsed,
and taken into account.
* Folders in the file system are now created for mailboxes on the server,
supporting hierarchical mailbox names.
* A connection worker now has an IMAPFolder that handles the local side of the
mailbox, and an IMAPMailbox that handles the server side.
* Connection workers are now created/deleted, and setup correctly. They will
now also wait in case they don't have a mailbox until they get some.

Initial work on a class that manages an IMAP folder on disk.

IMAP,POP3,NewMailNotification: fixed x86_64 build.

* Notifier::HeaderFetched(): fixed the method signature (go figure how it could
build on x86).
* POP3: fSizes type is now std::vector<size_t> instead of BList. Please review.
off_t might be a better choice.
* added the mail_daemon add-ons to the Haiku package again.

IMAP: downloading mails is now working.

* Changed the way the attributes are written to make sure that everything
that can be written once is in fact written just once.
* The rename code in BMailProtocol::_ProcessFetchedHeader() was broken,
and caused the hang of the last commit.

Mail Kit: Changed how filter changes work.

* BMailFilter::HeaderFetched() now only alters the entry_ref, and returns
B_MOVE_MAIL_ACTION to move a mail.
* Instead of potentially moving the file around several times, the
BMailProtocol now takes care of carrying out the filter action just once,
including trying to make the file name unique.
* This also allows the IMAP add-on to know the final location of the mail,
and thus downloading a message actually works.
* However, with my test inbox, it currently hangs, and a current Debugger does
not work on my older system -- I guess I need to update.
* Replaced the duplicated space mechanism within the "HaikuMailFormatFilter"
that is substantially faster, and handles all whitespace, not just spaces.
It will also replace tabs with spaces.

IMAP: work in progress of downloading the mail body.

* Most things are in place now, we just try to download the body to the
wrong file, as the final location is currently unknown.
* Added local only kPartialMessage flag for mails, but it's not being
used yet.

IMAP: call the folder's handler like the mailbox.

IMAP: IMAPFolder now stores the last UID.

* So mails are no longer downloaded every time.

imap: Fetching headers is now working.

* No state is stored yet, though, so that all messages will be downloaded
over and over again.

imap: WIP of getting fetching headers to work.

* Changed the FetchListener mechanism quite a bit.
* Doesn't seem to work correctly yet, although most pieces are in
place now.

imap: Work in progress of connection setup.

* SelectCommand now correctly encodes the mailbox name again.
* Hierarchy separators from the LIST/LSUB commands are now properly parsed,
and taken into account.
* Folders in the file system are now created for mailboxes on the server,
supporting hierarchical mailbox names.
* A connection worker now has an IMAPFolder that handles the local side of the
mailbox, and an IMAPMailbox that handles the server side.
* Connection workers are now created/deleted, and setup correctly. They will
now also wait in case they don't have a mailbox until they get some.

Initial work on a class that manages an IMAP folder on disk.