Using F prefix when trying to move into a wall or closed door yielded
"you attack thin air". Like the recently fixed F-vs-boulder case, give
more appropriate feedback. Also like F-vs-boulder, initiate digging if
wielding a pick-axe. (Also handles axes versus trees and closed doors).
One thing which isn't handled but possibly should be: F vs closed
door when not wielding a pick or other axe might attempt to force the door.
(Right now it gives "you harmlessly attack the door".)
aac603a446