Eliminate the cache that was supporting rndmonst() and pick a random
monster in a single pass through mons[] via "weighted reservoir
sampling", a term I'm not familiar with.
It had a couple of bugs: if the first monster examined happened to
be given a weighting of 0, rn2() would divide by 0. I didn't try
to figure out how to trigger that. But the second one was easy to
trigger: if all eligible monsters were extinct or genocided, it
would issue a warning even though the situation isn't impossible.
Aside from fixing those, the rest is mostly as-is. I included a bit
of formatting in decl.c, moved some declarations to not require C99,
and changed a couple of macros to not hide and duplicate a call to
level_difficulty().
Fixes#286
cbdda9dc9d