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being discovered; showing the terrain superseded showing the trap.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
b3580289be