crystal helmet

Discussed a long time ago, change helm of brilliance from iron to
crystal so that it doesn't need to be a special case for metallic
armor's affect on spell casting.  It now has a fixed description of
"crystal helmet" but is not pre-discovered.

Add new helm of caution to retain the "etched helmet" description
among the shuffled helms.  Wearing it confers the Warning attribute.
That's fairly lame but not necessarily useless.  It's iron and gets
half the former probability for a random piece of armor being helm
of brilliance so is not likely to be popular.

Helm of caution keeps the old helm of brilliance tile and new helm
of brilliance is basically the same image with different color and
pointed on the top.

Not changed:  the etched helmet was marked as green and is drawn
that way for text but the tile doesn't actually use green for it.
Crystal helmet started as hi_glass (rendered as 'bright cyan') but
has been changed to clr_white to match crystal plate mail.

Old save and bones files are invalidated.

doc/fixes3-7-0.txt

@@ -1154,6 +1154,8 @@ warnings via impossible() would be unseen if message suppression via ESC at
 wearing the Eyes of the Overworld overrides OPTIONS:blind; breaking the
 	always-blind conduct by doing that was intended but having permanent
 	blindness stay cured after removing them was not
+change helm of brilliance from iron to crystal so that it no longer needs to
+	be a special case for metallic armor vs spell casting
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository
@@ -1924,8 +1926,8 @@ boiling a pool or fountain now creates a temporary cloud of steam
 random themed rooms in the dungeons of doom
 extended achievement and conduct fields for xlogfile
 record amount of gold in hero's possession in xlogfile
-new objects:  amulets of flying and guarding, gold dragon scale mail and
-	set of gold dragon scales
+new objects:  amulets of flying and guarding, helm of caution,
+	gold dragon scale mail and set of gold dragon scales
 new monsters:  displacer beast ('f') and genetic engineer ('Q'), gold dragon
 	and baby gold dragon (both 'D')
 make camera flash which reveals previously unseen map features or objects or