cut trees and dug walls replaced by corridor symbols

cut trees and dug walls replaced by corridor symbols
- In various places, cutting down a tree would result in a corridor symbol
showing up, although trees are generally found in room-like areas.  Added
the missing checks.
- Trees were diggable except on mazelike levels.  Added the missing check.
- In a cavernous town like minetn-6, digging a wall would result in a
corridor symbol being displayed instead of a doorway.  This looked strange,
especially in the town.  Added a check for this, making use of the new
meaning of in_town(); non-town parts of this level still do CORR
replacement as expected.