Add a function to every game backend which indicates whether a game

Add a function to every game backend which indicates whether a game
state is in a solved position, and a midend function wrapping it.

(Or, at least, a situation in which further play is pointless. The
point is, given that game state, would it be a good idea for a front
end that does that sort of thing to proactively provide the option to
start a fresh game?)

[originally from svn r9140]