nethack -X vs normal game (trunk only)

Reported to the beta-testers list by <Someone> last April:
restoring a normal game save file in explore mode let you keep the file,
then after exploring and quitting without saving, you could restore it
again in normal mode and take advantage of whatever information you'd
gained.  This makes nethack -X (or playmode:explore) defer the switch to
explore mode when used while restoring a normal mode save file.  It now
performs a normal restore (with save file deletion) and then acts as if
the user had given the 'X' command interactively, requiring confirmation
to actually switch into explore mode.

src/options.c

@@ -4501,6 +4501,7 @@ set_playmode()
 	    wizard = FALSE;	/* not allowed or not available */
 	/* force explore mode if we didn't make it into wizard mode */
 	discover = !wizard;
+	iflags.deferred_X = FALSE;
     }
     /* don't need to do anything special for explore mode or normal play */
 }