bug: unit->busy flag doesn't get cleared

[Jim Kingdon <kingdon at panix dot com>]

The symptom: when you try to move a unit (including actions such as
"enter" but not things like Reserve which is not considered an
action), it will refuse to do anything.  Examining the state in a
debugger reveals that the unit->busy flag got stuck on (it is supposed
to be set when an action is queue and cleared when that action is
executed).  At the end of the turn, everything gets reset (per
explicit code which clears unit->busy then).

What makes this happen?  I saw it in the standard game.  The stuck
unit was always an infantry unit and there were always various bombers
about.  I think there was an "enter" action around the time it got
stuck, although I couldn't say for sure.

Can I reproduce it?  Not really.  I tried a test game in which I made
a bunch of bombers and infantry.  Sample configuration:

town contains:
  bomber, which contains:
    infantry
  bomber, which is empty.
  bomber, which contains:
    infantry
  infantry #3

Hit the "e" key for infantry #3 - sometimes this would cause infantry
#3 to lock up, but not always (or even usually).  I wonder if it was
timing-dependent or something.  Unfortunately, I did not turn on
debugging before I started, so I don't have a log to show from the
buggy case.

Possible way to workaround/ameliorate the situation: Make unit->busy
user-visible (like Reserve and flags like that).  This won't prevent
this bug from arising, but it will make it easier for the user to
figure out why the unit won't move.  Right now, it is totally
mysterious unless you happen to know about the bug.