Avoid a redirect loop in url-http

[Diane Murray <disumu@x3y2z1.net>]

On occasion url-http gets stuck in an endless redirect loop.  This can
be fixed by simply checking if the number of redirected URLs in
`url-callback-arguments' hasn't gone over a certain number (which
could be a customizable variable).  If it has been redirected too many
times, rather than call `url-retrieve-internal' again,
`url-http-parse-headers' should just let the callback function get
activated - after adding a redirect error to `url-callback-arguments',
so that the callback knows that the maximum number of redirects was
reached.

I've fixed this on my system, but I'm not including the changes I
made.  I'm currently waiting to get the Emacs copyright release form
(the FSF copyright-clerk has confirmed that the papers will be sent),
and by the time I receive the papers, sign them, and return them, they
probably wouldn't arrive back before the tentative release date for
Emacs 22.  If you'd like me to send my patch, please let me know.

--
Diane Murray