(next-line): Move error signaling and special end of

(next-line): Move error signaling and special end of
line behavior into line-move so that next-line is symmetric with
previous-line when next-line-add-newlines is nil.

(line-move): Move as far as possible and ding with a message
if the requested motion cannot be accomplished.  When
selective-display is numeric, ensure point actually moves and does
so to a visible line.