Clean up output destinations in navigation code

Clean up output destinations in navigation code

shp_nav() and shp_nav_one_sector() printed both to their actor
argument and to ship owner.  shp_nav_one_sector()'s use of xyas()
looked particularly suspicious: it passed actor, then printed the
result to the ship owner.  Fortunately, actor and ship owner are the
same, since even deities can't navigate foreign ships.  Normalize to
actor for consistency.

While there, rename shp_mess() to shp_stays().