For an explosion caused by the player while the hero is engulfed,
skip adjacent monsters. Also, don't give "you hear a blast" when an
unseen explosion is caused by a scroll of fire (explosion is always unseen
when hero is engulfed, regardless of explosion's cause). [Offhand I'm
not sure whether something other than the player can trigger an explosion
while the hero is engulfed.] Lastly, round up instead of down when a
monster takes half damage, so that non-zero can't be reduced to zero.
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