[physfs] Two Linux problems

[Ryan C. Gordon]

> I'm not suggesting that I'm trying to use the ~ shorthand in my
> program, of course I would use PHYSFS_getUserDir for that.  What I
> meant to say is that if the user has a PATH environment variable which
> uses ~ in some of the components (for example, by setting it in their
> .bashrc), physfs will not search those directories when looking for
> the executable.

I didn't realize that was legal (for the reasons Thorbjørn mentioned), 
but if, say, execvp() handles it, we should too. I'll make a note to 
look into this.

--ryan.