refine blocked levitation/flight (trunk only)

Levitation side-effects get skipped if Levitation toggles while it
is blocked, so BFlying (the reason Flying is blocked) could become stale
in some situations.  Enlightment feedback about latent flight capability
was the only thing affected.

include/extern.h

@@ -1750,6 +1750,7 @@ E void NDECL(self_invis_message);
 /* ### polyself.c ### */
 
 E void NDECL(set_uasmon);
+E void NDECL(float_vs_flight);
 E void NDECL(change_sex);
 E void FDECL(polyself, (int));
 E int FDECL(polymon, (int));