v4.

- Dropped the request-tracking POC stuff because those patches only
  benefit the Org integration, which needs special attention anyway.

- Added unifying changes to url-irc and browse-url that treat ERC and
  rcirc equally.


Questions (for anyone):

  1. I added a couple autoloads in lisp/url/url-irc.el to avoid having
     to create a url-ircs.el (or even a url-irc6{,s}.el). Is there a
     better alternate means of getting `url-scheme-get-property' to
     discover handlers that doesn't rely on autoloads?

  2. In the function `url-irc', I bind `url-current-object' around the
     call to `url-irc-function' to avoid adding another parameter to the
     latter's interface (which mainly benefits ERC). Any obvious problem
     with borrowing `url-current-object' for this purpose?

  3. In browse-url, I basically ignore what looks like the favored
     practice for adding handlers, namely, registering an internal
     function with `browse-url-default-handlers' that calls a public
     function assigned to a user option. An example of this pattern is:

       internal: browse-url--mailto
       option:   browse-url-mailto-function
       public:   browse-url-mail

     The reason for sidestepping the intervening indirection and adding
     a public function directly to `browse-url-default-handlers' is that
     I figure users wishing to override this can already do so via
     `browse-url-handlers'. Is that misguided somehow?

  4. Are any of these non-ERC changes newsworthy enough for etc/NEWS?

Thanks!