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/were.c

@@ -123,7 +123,7 @@ new_were(struct monst *mon)
         mon->mcanmove = 1;
     }
     /* regenerate by 1/4 of the lost hit points */
-    mon->mhp += (mon->mhpmax - mon->mhp) / 4;
+    healmon(mon, (mon->mhpmax - mon->mhp) / 4, 0);
     newsym(mon->mx, mon->my);
     mon_break_armor(mon, FALSE);
     possibly_unwield(mon, FALSE);