Having one of these makes it much easier to debug what's going on when
the solver can't solve something. Also, now the solver can grade the
difficulty of a puzzle, it's useful to expose that feature in a
command-line tool.

Tracks's previous loop detector was very basic, and only bothered to
highlight one loop, even if the player managed to create more than one
at a time. Now we highlight all of them.

Each game's .R file should append the binary to $(GAMES) in both the
'am' and 'gtk' makefiles, but presumably due to copy-and-paste from an
out-of-date version, tracks.R only did 'gtk'.

Also clarified CHECKLST.txt in the hope that I won't miss that again.