[AD] Splitting filesystem APIs

[Peter Wang <novalazy@xxxxxxxxxx>]

I'm quite sick of discussing this and you probably are all as well.
Just to let you know:

Tomorrow (later today) I plan to split up the current fshook API into
the traditional divisions: one part to deal with file streams, and one
part deal with directory hierarchies.  A new type, ALLEGRO_FILE, will
correspond to FILE.  For most users, including all uses in the addons
right now, that's the only difference they will see.

I will follow Elias's suggestion for a thread-local setting for the
current vtable, integrated with al_(re)store_state.  I won't pursue
my URI proposal.  Of course, it could be implemented by setting the
vtable.

Then I will concentrate on fixing the corner cases in the stream
operations, and leave the directory stuff to someone who cares.

Peter