BeOS support.

[Ryan C. Gordon]

I've added support for BeOS to PhysicsFS. This required me to split part
of platform/unix.c out into a new file: platform/posix.c.

It's not precisely POSIX compliant, I'm sure, but it works.
platform/unix.c and the new platform/beos.cpp (cpp! BeOS's API is object
oriented! How odd!) both use the common routines in the posix module.

After I get some sleep I'm going to package this up as an official
PhysicsFS 0.1.6 release. For now, everything is in CVS.

0.1.6 will be the last unstable release before 1.0.0. All that's left is
patching some bugs and other minor things.

--ryan.