mon->mhp manipulation

I've hunted for other instances where monster hit points were set
to zero or less without calling the routine that kills off the
monster (see recent mon_unslime() vs zhitm()) and didn't find any
for mhp subtraction.  I haven't checked for direct assignment yet.

For a while I thought I'd found several cases where a monster was
intended to be killed but got left with positive hit points, but
it turned out that lifesaved_monster(), of all places, was setting
them to zero.  I've moved that to its callers so that it isn't so
well hidden.  And changed several ''if ((mon->mhp -= dmg) <= 0)''
into separate subtraction and 'if' just so the mhp manipulation is
a bit more visible.

I think the only actual change here is the message for monster
being killed by lava, where glass golems now melt instead of burn.

src/makemon.c

@@ -770,6 +770,7 @@ xchar x, y; /* clone's preferred location or 0 (near mon) */
     /* Max HP the same, but current HP halved for both.  The caller
      * might want to override this by halving the max HP also.
      * When current HP is odd, the original keeps the extra point.
+     * We know original has more than 1 HP, so both end up with at least 1.
      */
     m2->mhpmax = mon->mhpmax;
     m2->mhp = mon->mhp / 2;