fog cloud and aquatic monsters

fog cloud and aquatic monsters
<Someone> pointed out the inappropriate appearance of the 3.4.2 message when
fog clouds have you engulfed if you happen to be polymorphed into an aquatic
creature.  It does seem that a fog cloud should not cause added damage to
amphibious creatures, which includes breathless (but non-flaming) monsters.
I also thought another message was more appropriate for flaming creatures
(see on_fire()), and added a special case for that.
gulpum was missing symmetric code and I added it there too.