Fix directory and naming of savestates, srm files.

These are patches for this issue here: https://github.com/libretro/RetroArch/issues/1030#issuecomment-55810822

1.) To sum it up: the recommendation in the issue report is: If a file /path/to/file.7z#folder/rom.img is loaded, the srm and state should be called rom.state and rom.srm
2.) Additionally, unless a savestate path is set, the directory to try to save them in should be /path/to/

I interpreted it like this now
    dirname(/path/to/file.7z#folder/rom.img) is /path/to 
    basename(/path/to/file.7z#folder/rom.img) is rom.img
    Then g_extern.basename is dirname(path) + basename(path)

Don't pull unless you agree with my basename and dirname definitions!

If this shouldn't be changed globally (which I think it should because it makes a lot of sense), I will just change the g_extern.basename in case of COMPRESSION and not the basic basename functions.