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.