Treat previous/next-line specially in erc-fill-wrap

Treat previous/next-line specially in erc-fill-wrap

* lisp/erc/erc-fill.el (erc-fill-wrap-visual-keys): Mention option
`erc-fill-wrap-force-screen-line-movement' in doc string.
(erc-fill-wrap-force-screen-line-movement): New option to suppress
logical-line movement with `previous-line' and `next-line' when
`erc-fill-wrap-mode' is enabled.
(erc-fill--wrap-move): Accept trailing args.
(erc-fill--wrap-previous-line, erc-fill--wrap-next-line): Use
`erc-fill--wrap-move', like all the other `fill-wrap' commands.
(Bug#60936)