[physfs] bug with group (or world) writable permissions

[Ryan C. Gordon]

> Forget it :))
> I'm a jacka..
> I guess the extrernal dependency is zlib ;^)

"external dependency" isn't accurate, I meant unzip.c, which was
seriously inefficient. Reimplementing the zip archiver in zip.c was a
better decision overall.

zlib is still needed, but we include it with the source, and the build
system can compile it in or use the system's preinstalled version. Getting
rid of that would be nontrivial for no good reason, since it's so easy to
include the complete code.

Short of the C library, there shouldn't be any required code that doesn't
come with physfs.

--ryan.