plug fountain & throne SIGHUP cheat

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.