fix github issue #1017 - webs in/on water

Reported by copperwater, it was possible for a web to be created
at a water or lava location.  It would not be displayed even after
being discovered; showing the terrain superseded showing the trap.
But it functioned normally and could trap the hero.  Webs pull the
victim to the floor so hero would drown or burn up on next move
even if the spot had been reached while floating or flying.

A monster spider couldn't survive at a water or lava location, but
a poly'd hero could.  Creating a web via #monster or wizard mode
wish could result in it affecting some unsuspecting player via bones.

Disallow creation of webs at water, lava, and air or cloud spots.
(They're already disallowed at furniture spots.)

Fixes #1017

doc/fixes3-7-0.txt

@@ -1162,6 +1162,8 @@ fake player monsters in endgame had a 1 in 1000 chance to be given a stack
 avoid "wall_angle: unknown hwall mode 4" when using #terrain command to
 	display a spot remembered as a wall but actually a secret door that
 	has been transformed into an open door while not in sight
+it was possible to produce a web at a water or lava location; it would not be
+	visible but could trap the hero, resulting in drowning or incineration
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository