digging prompt: > vs < (trunk only)

The intersection of a couple of recent patches:  noticed when
updating movement as a grid bug, and accentuated when fixing the attempt
to move down while levitating.  If you can't reach the floor due to
levitation, don't show '>' in the list of likely candidate directions for
the prompt when digging.  It was always included so that the list could
never be empty, but it's a poor suggestion to the player when levitating.
Use '<' instead in that situation; also a poor suggestion, but silly
rather than unintentional.  :-)

     This only affects the bracketted part of the "In what direction?"
prompt, not the actual digging (and player can pick any direction whether
it's shown in the prompt or not, so the digging code is already prepared
to handle attempts to dig up as well as down anyway).

doc/fixes35.0

@@ -305,6 +305,7 @@ prevent poly'd shopkeepers from taking on forms that can't handle objects
 attempting to move direction 'u' as a grid bug performed #untrap command;
 	the other diagonals reported "unknown command" instead of "you can't"
 mimic posing as statue or corpse now picks and maintains particular monst type
+when levitating, don't show '>' as a likely direction for digging
 
 
 Platform- and/or Interface-Specific Fixes