[physfs] Mounting Archive from within an archive

[Ryan C. Gordon]

> My idea is to mount such a ZIP to" missions/" and then open the HOG whenever 
> it's needed.

This isn't supported at the moment, but archives-in-archives is on the 
TODO list for the next major PhysicsFS revision.

> Even if I cannout mount the file I can check pretty good if it is 
> located in an archive or in physical form in the "missions/" directory. 
> If it's in an archive I probably just read out the HOG and write it to 
> "missions/" physically. In short: extracting the file. If the mission is 
> unloading, this temporary HOG could be removed/deleted/overwritten.

This is probably the best bet in the short term until this is implemented.

--ryan.