[physfs] Documentation improvements

[Jason McKesson]

I have made a patch (against Mercurial revision number 17bcf6924240) 
which improves the Doxygen-generated documentation for the library. It 
does several things:

1: It upgrades the Doxyfile from Doxygen version 1.3.4 to 1.7.2.
2: It adds a layout file that alters how the HTML source is laid out.
3: It pulls the \mainpage out into a separate file (docs/pages.doxy). 
This way, the main header doesn't have a giant blob of text in it.
4: It puts all of the functions into various modules. I find that 
organizing functions by module makes it *much* easier to find something.

Obviously, this requires a more recent version of the Doxygen generator 
(namely 1.7.2 or above, though it might work with some prior versions). 
None of the CMake build or anything else is affected by this.

The patch is attached. Comments are welcome; I would certainly like to 
see this incorporated into PhysFS.
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