Cleanups and improvements for FFAP and URL.

Cleanups and improvements for FFAP and URL.

* ffap.el (ffap-url-unwrap-local): Make it work right.
Use url-generic-parse-url, and handle host names and Windows
filenames properly.
(ffap-url-unwrap-remote): Use url-generic-parse-url.
(ffap-url-unwrap-remote): Accept list values, specifying a list of
URL schemes to work on.
(ffap--toggle-read-only): New function.
(ffap-read-only, ffap-read-only-other-window)
(ffap-read-only-other-frame): Use it.
(ffap-fixup-url): Don't check ffap-ftp-regexp, since it is not
necessary for ffap-url-unwrap-remote.

* url-parse.el (url-path-and-query, url-port-if-non-default): New
functions.
(url-generic-parse-url): Don't set the portspec slot if it is not
specified; that is what `url-port' is for.
(url-port): Only require the scheme to be specified to call
url-scheme-get-property.

* url-util.el (url-encode-url): Use url-path-and-query.

* url-vars.el (url-mime-charset-string): Load mm-util lazily.

Fixes: debbugs:9131