[physfs] PhysFS and absolute paths

[Brian Hook]

> Perhaps we should put in a toggle like we already have for symlinks?

That's what I was thinking, it would be pretty much orthogonal to 
that.  While I know that PhysFS is designed for games, adding this 
ability would seem to be relatively trivial (in terms of 
space/complexity), and allowing the same cross-platform file system 
access for both tools and code would seem to be a win-win.  Otherwise 
the game uses PhysFS, and the tools end up using fopen/etc. and have 
to deal with a different set of portability problems.

Brian