Wall spells destroyed at entrances

Added a new function to rooms that is called by players entering an
area. It destroys walls in a small radius around the entering player,
eliminating the 'trick' aspect of wall spells.

This setup allows walls to be used on exits to block escapes, but does
not allow walls to block entrances. Walls will still function normally
anywhere else in the area, and can still block small passages and the
like, but players will no longer 'stumble' into them.

I believe this is the best solution to wall spells. This requires no nerfs or balance changes; in fact, wall spells should probably be boosted in effect or size to compensate for their new inability to block room entrances.

Walls will be deleted by players entering through any method - Elusion, doors, zone edges - but not by logging on. That will allow walls to continue 'holding ghosts' and prevent mules from logging on and off to clear walls at non-entrance locations.

The radius of deletion is about 2 character-widths. In testing, a radius of 1 was useless, and a radius of 3 was enormous. I chose not to include the deletion radius as a setting unless requested - I don't think it's the kind of thing that really benefits from adjustment.