Make mailcap-prefer-mailcap-viewers work as documented

Make mailcap-prefer-mailcap-viewers work as documented

* lisp/emacs-lisp/seq.el (seq-find): Autoload.

* lisp/net/mailcap.el (mailcap-parse-mailcaps): Note where all the
entries come from so that we can later distinguish between user
values and system values (bug#36771).
(mailcap-parse-mailcap): Take a source parameter.
(mailcap-possible-viewers): No need to sort wildcards/exact
matches; these are later sorted anyway.
(mailcap-add-mailcap-entry): Remove `after' parameter.
(mailcap-mime-info): Make mailcap-prefer-mailcap-viewers work as
documented.