Patch from Mark Wooding (though somewhat tampered with by me): have
Loopy mark LINE_NO grid edges with very faint lines, instead of
leaving them totally undrawn. Helps in complex grid types where the
line layout isn't entirely obvious to a player not already familiar
with it. Disableable by setting LOOPY_FAINT_LINES=n in the
environment, just in case anyone turns out to seriously dislike it.
(You could probably disable it via LOOPY_COLOUR_6 too, but you'd
have to know the exact shade of your background to get that right.)

[originally from svn r8597]