eel hiding behavior (trunk only)

From a bug report:  an exposed eel in an isolated
pool--swamp rooms sometimes produce those--would never re-hide no matter
how long you left its vicinity.  Re-hiding is part of post-move handling
and an eel with no adjacent water to move into would never reach that bit
of code since it didn't move anywhere.  The code used to re-hide monsters
when you return to a previous level was ignoring eels altogether.  Give
unhidden eels a chance to hide earlier during monster movement and also
when the hero returns to their level.

     Also a really minor bit to slow down hit point loss of eels out of
water.  I don't think anybody reverse genocides krakens to get experience
any more since they don't provide a big bonus when they're out of water,
so this change won't have much of an affect.

doc/fixes35.0

@@ -281,6 +281,7 @@ changing alignment type resets alignment record to 0 (nomimally aligned)
 while polymorphed, suppress attribute gain/lose earned by pre-poly exercise
 wizard mode #monpolycontrol prompting asked about "it" when monster was unseen
 reprompt if player fails to make a menu choice during inventory identification
+seen eels who were stuck in isolated pools would never re-hide
 
 
 Platform- and/or Interface-Specific Fixes