Change gnutls-verify-error to be first-match

Change gnutls-verify-error to be first-match

* doc/misc/url.texi (Customization): Describe the new user
option url-lastloc-privacy-level.

* lisp/net/eww.el (eww-render): Set url-current-lastloc to the
url we are rendering, to get the referer header right on
subsequent requests.

* lisp/url/url-http.el (url-http--get-referer): New function
to determine which referer to send, if any, considering the
users privacy settings and the target url we are visiting.
(url-http-referer): New variable keeping track of the referer
computed by url-http--get-referer
(url-http-create-request): Use url-http-referer instead of the
optional argument to set up the referer header.  Leave
checking of privacy settings to url-http--get-referer.
(url-http): Set up url-http-referer by using
url-http--get-referer.

* lisp/url/url-queue.el (url-queue): New struct member
context-buffer for keeping track of the context a queued job
started from.
(url-queue-retrieve): Store the current buffer in the queue
object.
(url-queue-start-retrieve): Make sure url-retrieve is called
in the context of the original buffer, if available.

* lisp/url/url-util.el (url-domain): New function to determine
the domain of a given URL.

* lisp/url/url-vars.el (url-current-lastloc): New variable to
keep track of the desired "last location" (referer header).
(url-lastloc-privacy-level): New custom setting for more
fine-grained control over how lastloc (referer) is sent to
servers (Bug#27012).