Show mon vs mon attack messages only when target is visible

Show mon vs mon attack messages only when target is visible

The visibility check in mattackm doesn't guarantee both the
attacker and defender can be seen by hero. Before giving
messages, check more strictly whether we could see the
message happening - either really seeing the monster or
sensing the monster by some other means, depending on
the message.  This should remove most of the "It" messages.

Also unhide mimics who get gazed by umber hulks.  We could
keep the mimic hidden and make the messages reflect the
hulk gazing at the thing the mimic is mimicing, but this
is much easier.  This fixes bz631 / H4500