zlisp: improve `forward-line` implementation.

* lib/zemacs/specs/zlisp_move_spec.yaml (forward-line): Specify
complete behaviour of `forward-line`.
* lib/zile/buffer.lua (forward_line): Implement specified
behaviours.
* lib/zemacs/zlisp/move.zl (forward-line): Rewrite over
forward_line. Update docstring from Emacs.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>