Moved default renderer name into a #define.
Ported some filesystem changes from MP to SP.

Mainly the code to do with fs_homepath, so save files are now saved in the
user's home directory. I think this means SP config files are also saved
in the home directory as well.
Windows now uses Sys_LoadDll to load renderer.

Also copied across the fix from MP to unload the renderer library when it
gets shutdown.
Unregister window class when shutting down GLimp.

This is a fix ported over from MP again.