Rewrite choose_stairs(). Use of '&& !builds_up()' was a no-op in
normal branches and forced picking the down stairs/ladder in Vlad's
Tower (or Sokoban) rather than reversing the usual up/down choice.
Also, the logic used was backwards: Kops always surrounded the up
stairs instead of the down stairs as intended. Non-Kops picked the
opposite direction of what the arguments asked for but since they based
their choice of up versus down on hidden mon->m_id, the reversal wasn't
noticeable.
Extend the choosing so that if nothing in the requested direction can
be found, it tries the opposite direction. Otherwise when Vlad's m_id
happens to force the direction to be 'up', he has nowhere to teleport
to now that being in his Tower doesn't force 'down' anymore. He goes
to down ladder when on the top level but if you level teleport to the
next level down and he accompanies you, he might go to either the down
ladder or the up ladder when he tries to get away from you to heal.
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