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.

include/extern.h

@@ -2090,6 +2090,7 @@ E void FDECL(mselftouch, (struct monst *,const char *,BOOLEAN_P));
 E void NDECL(float_up);
 E void FDECL(fill_pit, (int,int));
 E int FDECL(float_down, (long, long));
+E void NDECL(climb_pit);
 E int FDECL(fire_damage, (struct obj *,BOOLEAN_P,BOOLEAN_P,XCHAR_P,XCHAR_P));
 E void FDECL(water_damage, (struct obj *,BOOLEAN_P,BOOLEAN_P));
 E boolean NDECL(drown);