fix #1727 - offering Rider corpses (trunk only)

fix #1727 - offering Rider corpses (trunk only)

     The report that killing a Rider on an altar allowed you to sacrifice
its corpse was a wizard mode-only phenomenon in 3.4.3 (because you needed
to use an altar at some location other than the Astral Plane, hence also
needed to use ^G to get the Rider there), so not really a bug.  But a
post-3.4.3 change has made it possible to offer corpses from the floor on
the astral level, unintentionally making it possible to sacrifice Rider
corpses.  This makdes #offer check specifically for them and trigger their
revival if the attempt is made, same as done by pickup.