adopt pull request #328 - steam clouds

This adds a superset of the code from github pull request #328
to create a short-lived cloud of steam when fire hits a pool or
fountain.  The original code required C99; this doesn't.  It also
allowed vapor clouds on the Plane of Water where they don't work
sanely becaure regions don't understand air bubble movement and/or
vice versa.  This inhibits the clouds there [the same ought to be
done for scrolls of stinking cloud].  It also left as-is the code
that reported when fountains got used up even though conceptually
the steam should interfere with being able to see that.  This adds
a glyph-check hack to augment cansee() so that fountains that boil
away entirely are hidden at the time.

Regions that block line of slight should be calling block_point()
when created and unblock_point() when removed but a naive attempt
to introduce that didn't work as expected so I'm giving up on.

Fixes #328

doc/fixes37.0

@@ -268,6 +268,7 @@ wizard mode #wizborn command
 include more skill information in ^X output when dual-wielding
 item-using monsters will zap wand of undead turning at corpse-wielding hero
 	when the corpse is harmful
+boiling a pool or fountain now creates a temporary cloud of steam
 
 
 Platform- and/or Interface-Specific New Features