fix gibhub issue #320 and more - 'mention_decor'

Fixes #320

Avoid giving "you are back on the bottom" nearly every step when
moving around underwater.

Avoid "you are back on floor" followed by "you trip over <object>"
when fumbling in case that fumbling was due to being on ice when
taking the step to floor.  Done for all fumbling rather than just
one-turn fumbling instigated by ice.

When moving from ice or water to ground, show "you are back on floor"
before listing objects at that spot instead of after.

I think there was at least one more thing but have lost track.  At
any rate, 'mention_rate' potentially has a new set of bugs.

include/flag.h

@@ -257,6 +257,7 @@ struct instance_flags {
     boolean autodescribe;     /* autodescribe mode in getpos() */
     boolean cbreak;           /* in cbreak mode, rogue format */
     boolean deferred_X;       /* deferred entry into explore mode */
+    boolean defer_decor;      /* terrain change message vs slipping on ice */
     boolean echo;             /* 1 to echo characters */
     boolean force_invmenu;    /* always menu when handling inventory */
     boolean hilite_pile;      /* mark piles of objects with a hilite */