gnu: Rename "gnubackgammon" back to "gnubg".

This reverts c97e4d8b, which was not
consensual and broke 'tests/guix-package.sh'.

* gnu/packages/games.scm (gnubackgammon): Rename to...
(gnubg): ... this, and rename the 'gnubg' deprecated alias to
'gnubackgammon'.