report ice's thaw state

Classify nearby ice as "solid" (no melt timer), "sturdy" (more than
1000 turns left), "steady" (101 to 1000 turns left), "unsteady" (51
to 100 turns left), "thin" (15 to 50 turns left), or "slushy" (1 to
14 turns left, matching walking on ice with the Warning attribute).
[I'm not thrilled with "steady" and particularly "unsteady".]

I was originally going to do this just for probing downward, but ended
up also doing it for look-here and getpos's autodescribe.  It nearly
got out of hand and touched more files than anticipated.

'mention_decor' ought to treat moving from ice firmer than thin to
thin or slushy, from thin to slushy, from slushy to any other, and
from thin to firmer as if moving onto different terrain but I haven't
attempted to tackle that.

The melt timer could work more like a candle's burn timer, triggering
at intermediate stages and resetting itself, so that ice which changes
to a weaker state under the hero could be reported to the player.  But
this doesn't implement that.

include/flag.h

@@ -1,4 +1,4 @@
-/* NetHack 3.7	flag.h	$NHDT-Date: 1684791761 2023/05/22 21:42:41 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.217 $ */
+/* NetHack 3.7	flag.h	$NHDT-Date: 1698264779 2023/10/25 20:12:59 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.224 $ */
 /* Copyright (c) Stichting Mathematisch Centrum, Amsterdam, 1985. */
 /*-Copyright (c) Michael Allison, 2006. */
 /* NetHack may be freely redistributed.  See license for details. */
@@ -290,6 +290,7 @@ struct instance_flags {
     boolean zerocomp;         /* write zero-compressed save files */
     boolean rlecomp;          /* alternative to zerocomp; run-length encoding
                                * compression of levels when writing savefile */
+    schar ice_rating;         /* ice_descr()'s classification of ice terrain */
     schar prev_decor;         /* 'mention_decor' just mentioned this */
     uchar num_pad_mode;
     uchar bouldersym;         /* symbol for boulder display */