steeds don't remember traps

<Someone> reported to the list that steeds didn't remember traps
encountered while mounted.  When not mounted, a monster will remember
traps, even when they don't damage the monster.  To that end, added code to
set the steed's mtrapseen mask.

doc/fixes34.1

@@ -382,6 +382,7 @@ fix sequencing of Magicbane's hit messages
 avoid buffer overflow from long or too many -s params
 wake up first if trying to crawl out of water while asleep
 while waiting, don't try to change into were form when already in were form
+steed should remember traps encountered while mounted
 
 
 Platform- and/or Interface-Specific Fixes