use .IsSet() instead if .Get() != NULL

Change-Id: Ia2b7a719fd398e78cc3b11d4f7b02cb81179f65f
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3488
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>

Storage Kit: Style fixes to mime database

* Update doxygen docs a bit, especially returns.
* Some variable renaming for consistency and clarity, err/error => result.
* Remove some dead code that isn't coming back. (code moved to IconUtils)
* OpenType() returns a status_t, put it in a status_t, not a ssize_t.
and then later create a new ssize_t to hold the read bytes read
and use that, saves a lot of casting and confusion.

Resolve #10435.

Adjust Database{Location} to only attempt to create a mimetype when
actually necessary, and fail otherwise if a writable version doesn't yet
exist. Correspondingly, adjust callers such as
DatabaseLocation::DeleteAttribute(). Fixes a problem where a caller asking
to perform a mimeset could fail early due to SetSupportedTypes() attempting
to update the read-only mime database entry supplied by a package, and
consequently most of the mimeset operations would be skipped.

Fix MIME type creation error

DatabaseLocation::OpenOrCreateType(): BNode::WriteAttr() returns the
number of bytes written, not just an error code., so we erroneously
reported an error on success.

Fixes the first part of #9845.

storage/mime: Some host platform build fixes

Refactor MIME DB access

* Add class DatabaseLocation. It contains a list of the MIME DB
directory paths plus methods to access type files.
* Move all low-level MIME DB access functions from
database_{support,access} to DatabaseLocation. All code that formerly
used those now requires a DatabaseLocation object. In BMimeType and in
the registrar the default object is used, but the low-level classes
can now be reused with different locations.
* Move get_icon_data() from database_access to database_support and
delete the former, which is now empty.

Storage Kit: Style fixes to mime database

* Update doxygen docs a bit, especially returns.
* Some variable renaming for consistency and clarity, err/error => result.
* Remove some dead code that isn't coming back. (code moved to IconUtils)
* OpenType() returns a status_t, put it in a status_t, not a ssize_t.
and then later create a new ssize_t to hold the read bytes read
and use that, saves a lot of casting and confusion.

Resolve #10435.

Adjust Database{Location} to only attempt to create a mimetype when
actually necessary, and fail otherwise if a writable version doesn't yet
exist. Correspondingly, adjust callers such as
DatabaseLocation::DeleteAttribute(). Fixes a problem where a caller asking
to perform a mimeset could fail early due to SetSupportedTypes() attempting
to update the read-only mime database entry supplied by a package, and
consequently most of the mimeset operations would be skipped.

Fix MIME type creation error

DatabaseLocation::OpenOrCreateType(): BNode::WriteAttr() returns the
number of bytes written, not just an error code., so we erroneously
reported an error on success.

Fixes the first part of #9845.

storage/mime: Some host platform build fixes

Refactor MIME DB access

* Add class DatabaseLocation. It contains a list of the MIME DB
directory paths plus methods to access type files.
* Move all low-level MIME DB access functions from
database_{support,access} to DatabaseLocation. All code that formerly
used those now requires a DatabaseLocation object. In BMimeType and in
the registrar the default object is used, but the low-level classes
can now be reused with different locations.
* Move get_icon_data() from database_access to database_support and
delete the former, which is now empty.