ncplane_destroy() needs to call ncplane_reparent_family(), not
ncplane_reparent() as it was doing (closes#1291). ->absy and
->absx actually are absolute; return them directly for an O(1)
ncplane_abs_yx() (down from O(N), huzzah). Add some unit tests
related to destroying and reparenting (#1286). Add ncplane_abs_y()
and ncplane_abs_x(), document them, etc.