Bring up score dialog with multiple worms/game

Considering the min number of worms per game in two, I wonder what this
was ever about. There's a separate check below to make sure only one
human is playing.
Copy scores-backend.c and setgid-io.c from glines

For some reason we were stuck with a stub instead of a real copy of
games-scores-backend.

Conflicts:
	src/Makefile.am