Use a common funcion for all monster healing

Previously, the code for monster healing was repeated every time it
was needed; this commit sends it all through a common function, which
will make it easier to make changes to how monster healing works in
the future.

This is just a code reorganisation and won't have any gameplay
effect unless I made a mistake.

src/wizard.c

@@ -396,7 +396,7 @@ tactics(struct monst *mtmp)
         /* if you're not around, cast healing spells */
         if (distu(mx, my) > (BOLT_LIM * BOLT_LIM))
             if (mtmp->mhp <= mtmp->mhpmax - 8) {
-                mtmp->mhp += rnd(8);
+                healmon(mtmp, rnd(8), 0);
                 return 1;
             }
         FALLTHROUGH;