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.

I still don't actually have a solver design, but a recent conversation
sent my brain in some more useful directions than I've come up with
before, so I might as well at least note it all down.

This commit removes the old #defines of TRUE and FALSE from puzzles.h,
and does a mechanical search-and-replace throughout the code to
replace them with the C99 standard lowercase spellings.

I went through all the char * parameters and return values I could see
in puzzles.h by eye and spotted ones that surely ought to have been
const all along.

several different systems in strange directories. So I'm creating an
`unfinished' directory within source control, and centralising all
my half-finished, half-baked or otherwise half-arsed puzzle
implementations into it. Herewith Sokoban (playable but rubbish
generation), Pearl (Masyu - rubbish generation and nothing else),
Path (Number Link - rubbish generation and nothing else) and NumGame
(the Countdown numbers game - currently just a solver and not even a
generator yet).

[originally from svn r6883]