fix #K3231 - objects vs pits and holes

fix #K3231 - objects vs pits and holes

This got out of hand pretty quickly.  can_reach_floor() had
different criteria than trap activation.  Objects dropped at a
hole locations that don't fall through were treated as if they
were at the bottom of an abyss, so couldn't be examined or
picked up.

This a bunch of changes; it is bound to introduce some new bugs.