Check actual free disk space when games ask.

Windows only for the moment.
Check disk space on Posix/Android systems also.

Android apparently decided to skip statvfs() and only implement the older,
os-dependent statfs().
Warning fix.
Merge pull request #7036 from unknownbrackets/memstick

Check memory stick free space and tell the game about it
Merge https://github.com/hrydgard/ppsspp