Fix bit rot in the 'unfinished' subdir.

Fix bit rot in the 'unfinished' subdir.

Several of the source files here won't quite compile any more, because
of minor things like const-correctness and the UI_UPDATE change. Now
they should all build again (without prejudice to how useful they are
once they have built).

The biggest change was to remove the fatal() implementation from the
standalone path.c, because my new plan is that basically everything
that's not linked against a true puzzle frontend will be linked
against nullfe.c, which provides that function anyway.