kernel & add-ons: Adjustments to use DoublyLinkedList::InsertBefore.

The Insert(before, element) function has been marked deprecated since
2010, but still had many usage and kept accumulating more. It's long
past time we got rid of all them and actually deprecated the function
itself.

Insert(before, element) just calls InsertBefore, so no functional change.

packagefs: do not use auto-generated attributes for root node

The root node does not come from any package, so it shouldn't have the
SYS:PACKAGE_FILE attribute. Remove it from there.

Fixes #12453

Change-Id: Icb695bb90644753d4af1c4215c4ad3c2dd9c5e68
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5951
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

packagefs: Rename operator< to HasPrecendenceOver().

As suggested by weinhold on the mailing list.

packagefs: Rename isNewest to overridesHead.

As pointed out by Adrien. Thanks for the review!

Also fixed missing dereference operators.

packagefs: Move PackageNode comparison into an operator<.

This commit does not amount to any functional change: we still compare
purely by modification date alone. But it does de-duplify the code somewhat,
and makes it possible to easily implement changes to the comparison.

Fix reading attributes of directories

When reading an attribute of a directory there was no guarantee that the
underlying package would be open. When it wasn't reading an attribute
would fail, unless the attribute data were already cached. The reasons
for this are:
* UnpackingDirectory didn't forward the {Init,Uninit}VFS() calls to the
underlying PackageDirectory.
* Only PackageFile was actually opening the package in InitVFS().

Now we forward the {Init,Uninit}VFS() calls in all cases -- even in
{Add,Remove}PackageNode(), when the active package node changes -- and
opening/closing the package is now done in
PackageNode::{Init,Uninit}VFS().

packagefs: Share string instances globally

Introduce a class String which refers to shared private data that is
registered in a global hash table (in class StringPool) and use the
class consequently. This eliminates duplicate allocations for the same
string and also speeds up tests for equality. There's quite a bit
overhead for the management structures (apparently even more than for
the string data itself), but due to almost all strings being used
multiple times this still almost halves the memory usage for string
data.

packagefs: Organize sources into subdirectories

Fix reading attributes of directories

When reading an attribute of a directory there was no guarantee that the
underlying package would be open. When it wasn't reading an attribute
would fail, unless the attribute data were already cached. The reasons
for this are:
* UnpackingDirectory didn't forward the {Init,Uninit}VFS() calls to the
underlying PackageDirectory.
* Only PackageFile was actually opening the package in InitVFS().

Now we forward the {Init,Uninit}VFS() calls in all cases -- even in
{Add,Remove}PackageNode(), when the active package node changes -- and
opening/closing the package is now done in
PackageNode::{Init,Uninit}VFS().

packagefs: Share string instances globally

Introduce a class String which refers to shared private data that is
registered in a global hash table (in class StringPool) and use the
class consequently. This eliminates duplicate allocations for the same
string and also speeds up tests for equality. There's quite a bit
overhead for the management structures (apparently even more than for
the string data itself), but due to almost all strings being used
multiple times this still almost halves the memory usage for string
data.

packagefs: Organize sources into subdirectories