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.

src/sit.c

@@ -50,7 +50,10 @@ dosit()
 	}
 #endif
 
-	if(!can_reach_floor())	{
+	if (u.uundetected && is_hider(youmonst.data) && u.umonnum != PM_TRAPPER)
+	    u.uundetected = 0;		/* no longer on the ceiling */
+
+	if (!can_reach_floor()) {
 	    if (Levitation)
 		You("tumble in place.");
 	    else