[AD] physfs current directory

[Elias Pschernig <elias.pschernig@xxxxxxxxxx>]

Trent was wondering in #nooskewl how the recent "physfs: Implement
al_change_directory semantics. " patch works together with the PhysFS
search paths:

https://github.com/elias-pschernig/allegro5/commit/c6d76b1b653679dbbdd5987b3fb14472bae8d6f6

I haven't used PhysFS myself so may be misunderstanding. But the idea of
PhysFS seems to be that you do:

al_load_bitmap("sprite.png")

And PhysFS then goes through its list of search paths to find the file.
But after that patch physfs now is looking for a file "/sprite.png"
only. For example in ex_physfs, if I remove this line:

PHYSFS_addToSearchPath("data/ex_physfs.zip", 1)

Then put a file "02.png" into /tmp and run it as:

ex_physfs /tmp

It will fail to find the file, instead trying to open a
non-existing "/02.png".