U761 - Sitting as a hider and other related is_hider behavior

While looking at the behavior of sitting hiders, I noticed other related
odd behavior.
- player hiding while poly'd as a hider that hangs on the ceiling now drops to
  the floor before sitting (similar to the behavior of movement commands).
- trappers, as per data.base, don't hang on the ceiling.  Changed the
  mattacku case dealing with hiders to not treat trappers as ceiling hiders.
- updated can_reach_floor to also exclude ceiling hiders.  This covers a
  bunch of cases, such as pickup, look-here-while-blind, and so on.  Another
  alternative would have been to automatically unhide for all such cases.
trunk only, it's a minor bug IMO and we appear to be close to a release.

doc/fixes35.0

@@ -51,6 +51,9 @@ update display if bestowed a spellbook while unable to see invisible self
 use small pool of static buffers for mon_nam() and Monnam()
 Acknowledge Schroedinger's cat at end of game
 grammar fixes for applying stethoscope to corpses and statues
+player polymorphed as a ceiling hider cannot reach the floor, but
+	automatically unhide on #sit
+trappers do not hide on the ceiling
 
 
 Platform- and/or Interface-Specific Fixes