make #migratemons unconditional

Make #migratemons command be unconditional.  The show existing
ones part is now always present.  Compile-time DEBUG_MIGRATING_MONS
controls whether the create N new migrators part is available.

Fix creating new ones from the Castle level.  It knew how to find
the next level (Valley) but wouldn't send monsters there because of
its Is_botlevel() check.

dat/wizhelp

@@ -11,6 +11,7 @@ Debug-Mode Quick Reference:
 
 #levelchange    == set hero's experience level
 #lightsources   == show mobile light sources
+#migratemons    == show migrating monsters; optionally create some
 #panic          == panic test (warning: current game will be terminated)
 #polyself       == polymorph self
 #stats          == show memory statistics