new 'mention_decor' option

Somewhat similar to 'mention_walls', 'mention_decor' is a way to
request additional feedback when moving around the map.  It reports
furniture or unusual terrain when you step on that.  Normally stepping
on furniture only mentions it when it is covered by object(s).  And
moving onto (rather than into) water or lava or ice doesn't bother
saying anything at all.  With the new option set there will be a
message.  It uses Norep so won't repeat when moving from one water
spot to another or one lava spot to another or one ice spot to another
unless there has been at least one intervening message.  There is also
a one-shot message when moving from water or lava or ice onto ordinary
terrain (not Norep, just once since there's no land to land message).

Having the verbose flag Off doesn't inhibit these new messages but it
does shorten them: "A fountain." instead of "There is a fountain here."

The Guidebook gets a new subsection "Movement feedback" of the "Rooms
and corridors" section and it covers more than just 'mention_decor'.
As usual, Guidebook.tex is untested.

'mention_decor' persists across save/restore, so 'struct flags' has
changed and EDITLEVEL is being bumped, hence save files are invalided.

include/patchlevel.h

@@ -14,7 +14,7 @@
  * Incrementing EDITLEVEL can be used to force invalidation of old bones
  * and save files.
  */
-#define EDITLEVEL 11
+#define EDITLEVEL 12
 
 #define COPYRIGHT_BANNER_A "NetHack, Copyright 1985-2020"
 #define COPYRIGHT_BANNER_B \