Changed my mind about midend_is_solved: I've now reprototyped it as

midend_status(), and given it three return codes for win, (permanent)
loss and game-still-in-play. Depending on what the front end wants to
use it for, it may find any or all of these three states worth
distinguishing from each other.

(I suppose a further enhancement might be to add _non_-permanent loss
as a fourth distinct status, to describe situations in which you can't
play further without pressing Undo but doing so is not completely
pointless. That might reasonably include dead-end situations in Same
Game and Pegs, and blown-self-up situations in Mines and Inertia.
However, I haven't done this at present.)

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@9179 cda61777-01e9-0310-a592-d414129be87e