fire attack messages

Make sure the three instances of special fire effects stay
synchronized in the future by moving the relevant code into its own
routine.

     Shouldn't fire vortices and fire elementals also yield "already
on fire"?  How about ice vortices "melting"?

include/extern.h

@@ -1164,6 +1164,7 @@ E int FDECL(little_to_big, (int));
 E int FDECL(big_to_little, (int));
 E const char *FDECL(locomotion, (const struct permonst *,const char *));
 E const char *FDECL(stagger, (const struct permonst *,const char *));
+E const char *FDECL(on_fire, (struct permonst *,struct attack *));
 
 /* ### monmove.c ### */