plug fountain & throne SIGHUP cheat

When using a fountain or throne, getting a --More-- prompt
usually means that you're about to be told that it has just dried
up or vanished.  But since that topology change didn't occur until
after the message, players could cheat by forcing SIGHUP instead
of answering the prompt; the resulting save file would retain the
original topology.  So change the dungeon prior to telling the user
about it.

     The `fixes' entry possibly belongs in the tty-specific category
but since the change is to core code I put it in the general section.

doc/fixes34.1

@@ -157,6 +157,7 @@ applying a lance against a long worm could cause an impossible
 a knight applying a lance did not do a caitiff check
 blessed gain level when already at level 30 won't reduce experience points
 keep counting spell skill exercise even after expert status is reached
+when a fountain dries up or a throne vanishes, make sure it really happens
 
 
 Platform- and/or Interface-Specific Fixes