Improve url matching in ffap.el.

Improve url matching in ffap.el.

* ffap.el (ffap-url-regexp): Don't require matching at front of string.
(ffap-url-p): If only a substring matches, return that.
(ffap-url-at-point): Use the return value of ffap-url-p.
(ffap-read-file-or-url, ffap-read-file-or-url-internal)
(find-file-at-point, dired-at-point, dired-at-point-prompter)
(ffap-guess-file-name-at-point): Likewise.
(ffap-replace-file-component): Fix typo.

Fixes: debbugs:4952