Make message-mailto work for emacsclient

* doc/misc/message.texi (System Mailer Setup): Add index entry.
Mention option to use emacsclient.

* etc/NEWS: Mention emacsclient option for 'mailto:' handling.

* etc/emacs-mail.desktop: Add example using emacsclient.

* lisp/gnus/message.el (message-mailto): Add optional url argument
so we can call it from emacsclient.
3 jobs for master in 8 minutes and 45 seconds (queued for 1 second)
Status Job ID Name Coverage
  Fast
passed #13542
test-fast-inotify

00:06:37

 
  Normal
passed #13543
test-lisp-inotify

00:02:08

passed #13544
test-lisp-net-inotify

00:01:38