[minor] UNIX games path (Filesystem Hierarchy Standard)

This is minor, the game still plays fine.

normally, games executables go in prefix/games, not prefix/bin. The default could change, that's still easily controllable during compilation.

Also game data files should go in datadir/games/taisei, not datadir/taisei. That's not controllable during compilation. The other folders (application, icons, mime, doc) are in the correct place(datadir/XXXXX).