sleeping steeds (trunk only)

From the newsgroup:  it was possible to saddle, mount, and ride on a
sleeping jabberwork without it ever waking up.  Movement was checking for
timed sleep (!mon->mcanmove, set when mon->mfrozen contains a timer count
for either sleep or paralysis) but not indefinite sleep (mon->msleeping).
This moves the checking into its own routine which handles both types.
And it gives monsters a chance to wake up when they get saddled or mounted.

include/extern.h

@@ -2101,6 +2101,7 @@ E void NDECL(exercise_steed);
 E void NDECL(kick_steed);
 E void FDECL(dismount_steed, (int));
 E void FDECL(place_monster, (struct monst *,int,int));
+E boolean FDECL(stucksteed, (BOOLEAN_P));
 #endif
 
 /* ### teleport.c ### */