Refinement of the digging code:
* Picks should not chop down trees, but axes should.
* Picks should break walls, rock, statues, and boulders; axes shouldn't.
* Either picks or axes should chop down doors.
Can't push boulders through iron bars; traps can't roll such through either;
likewise for objects thrown by monsters.
Thrown objects susceptible to breaking might do so when they hit iron bars.
Assorted monsters can pass through iron bars; ditto for polymorphed character.
Attempting to dig iron bars will wake nearby monsters instead of yielding
"you swing your pick-axe through thin air".
Autodig won't accept iron bars as candidate location.
The autodig code still requires lots of interaction, because of all the
digging messages. Make these messages behave more like boulder pushing
messages, where they only display if the previous move wasn't a push.
