recursive spoteffects (trunk only)

recursive spoteffects (trunk only)

     Attempt to fix a buglist item:  if hero poly'd into iron golem form
enters a pool of water and drowning triggers reversion to human/whatever
form due to water damage, he will fall into that pool again, crawl or
teleport out, then redundantly crawl or teleport out for the initial entry.
[ spoteffects -> drown -> losehp -> rehumanize -> polyman -> spoteffects
  -> drown ]

     I don't have a lot of confidence in this fix.  It does handle the
reported problem, and hasn't broken a couple of earlier tricky cases
(ice melting to water, land mine turning into a pit).  But drown() and
lava_effects() seem to leave a trail of special case handling wherever
they appear so they--or spoteffects, or both--ought to be redone somehow.