polymorphing into a flyer while in a pit

<Someone> reported that if you polymorph into a flying monster while in a
pit, you must take u.utrap turns to first climb out before you can fly.  Of
course, once you're out, you can swoop down into the pit to pick things up
w/o delay.  Rather that have you automatically fly out (e.g. like quaffing
a potion of levitation), I thought it was better to take a turn to fly out,
so that's what I've implemented.

The code to deal with exiting a pit is moved to a new climb_pit function
and the "up" command now lets you climb from a pit too (something I've
found non-intuitive in the past).

Finally, I noticed that non-moving monsters could still go up/down even
though they couldn't move around.  Added non-moving checks in doup/dodown.

doc/fixes35.0

@@ -75,6 +75,8 @@ charge for reviving a shop owned corpse or reanimating a shop owned statue
 filled trap doors on castle can be re-dug
 message order when swapping places with a pet (e.g. into a trap), also use
 	different term instead of "displace"
+flyers can get out of pits more easily than non-flyers
+allow use of the < command to try to exit a pit
 
 
 Platform- and/or Interface-Specific Fixes