Remove door opening/closing from ctrl-dir; remove untrapping; code cleanup.

Untrapping is more or less never used; the only untrappable randomly
generated trap are webs, and those are safe enough to destroy by walking
in and out of.

Door opening is fine on normal direction key, and door closing can be done
via C just fine, for the fairly rare times that it is needed (which is also
just one keypress most of the time).

The changes are all so caught up with each other that it really wasn't
possible to split up into smaller commits, or at least by the time I
realised I was refactoring it was too late.
Zotdef: allow placing traps on top of traps (PleasingFungus).