ncdirect ought have relative moves

[dankamongmen]

We might not be able to portably determine where we are, but we can count how far we've come from where we've been. It would thus be useful to add cursor-relative moves to ncdirect. I see no real use for these in fullscreen mode (they're trivially synthesized if so from ncplane_yx() + ncplane_cursor_move_yx()).

this requires underlying terminal support, but i think that's pretty broad (we want e.g. cuu1, cuf1, cub1, and cud1). note that if we could somehow detect a failure, this would allow a means to determine absolute cursor position...