Accommodate ircs:// URLs in url-irc and browse-url

Accommodate ircs:// URLs in url-irc and browse-url

* lisp/url/url-irc.el (url-irc-function): Change signature of function
interface to expect a final "scheme" argument, such as "ircs".
(url-irc): Call `url-irc-function' with new positional argument, the
scheme extracted via `url-type' from the input URL.
(url-irc-erc, url-irc-rcirc, url-irc-zenirc): Accept a URL scheme as a
sixth positional arg.
(url-ircs-default-port, url-ircs): Add new autoloaded constant and
alias for `url-scheme-get-property' to recognize.  Do this to avoid
having to add another file.
* lisp/net/browse-url.el (browse-url-irc-function): Add new option.
(browse-url--irc): Add new function to call `browse-url-irc-function'.
(browse-url-default-handlers): Add "irc://" entry.
(browse-url-irc): Add new function to serve as general handler for
"irc://" URLS.  Accept trailing variadic args to accommodate
non-browse-url interfaces as well.
* test/lisp/net/browse-url-tests.el
(browse-url-tests-select-handler-irc): Add test for "irc://" URL
pattern.
* etc/NEWS: Mention select browse-url and url-irc
changes.  (Bug#56514.)