Adjust all struct dirent creations (again), this time to use offsetof().

The dirent struct is not packed, so offsetof(dirent, d_name) != sizeof(dirent).
Thus in order not to waste the alignment bytes (which are significant,
on x86_64 at least, sizeof(dirent)==32, but offsetof(...)=26.)

This is also the most portable way to handle things, and should
work just fine in cross-platform code that has a non-zero-sized d_name.

FAT: More miscellaneous cleanup.

* Remove __RO__ hack inherited from BeOS sample code.
* Remove and excise unneeded macros.
* Remove readlink, VFS will return EINVAL in its absence anyway.

FAT: Make more use of C++ features and utility classes.

This driver was fully C until relatively recently, when it was
switched to C++ so that it could be used with fs_shell. However,
it still is mostly using C paradigms; so this commit begins the
refactor away from that.

If I did this correctly, there should not be any functional change.

Change-Id: Id87d17b2e019ccdd1c7f07156cfe2a2124496675
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4725
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

file_systems & Tracker: Do not assume sizeof(dirent) contains 1 byte for the name.

At present, it does, but that is an oddity we have preserved from BeOS
that the next commit is going to remove. (This commit thus wastes 1 byte
without the following one.)

Most changes are pretty straightforward: only a +1 is needed,
and a few removed from sizing calculations. Some filesystems like UDF
originally passed back the length with the \0 included, so they have
been adjusted further. UFS2 had some other sizing problems which are also
corrected in this commit.

file_system/fat: Rework driver for fs_shell support

* Introduce fat_shell for build system fat manipulation
* Will theoretically let us do away with mtools when we
have another internal tool for partition manipulation

Change-Id: I661be556e79009842f157a9402c8f85da85d6336
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3556
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>