Check for client certificates when using GnuTLS

This fixes Bug#33780, and extends the documentation to describe how to
enable use of client certificates.

* lisp/net/network-stream.el (network-stream-certificate): Correct
order of parameters to plist-get.
(network-stream-open-tls): Pass all received parameters to
open-gnutls-stream as plist, not just :nowait.

* lisp/net/gnutls.el (open-gnutls-stream): Change optional nowait arg
to be plist.  Derive nowait and client certificate(s) and keys(s) from
plist (maybe via auth-source) and pass to gnutls-boot-parameters and
gnutls-negotiate.
(network-stream-certificate): Add declare-function form for it.

* doc/misc/auth.texi (Help for users): Describe format to use for
client key/cert specification.

* doc/misc/emacs-gnutls.texi (Help For Developers): Describe usage of
optional plist argument.  Add crossreference to description of
.authinfo format for client key/cert specification.

* etc/NEWS: Describe new client certificate functionality for
  'open-network-stream'.

* test/lisp/net/network-stream-tests.el: Add require of network-stream.
(connect-to-tls-ipv4-nowait): Bind network-security-level to 'low
in order to bypass nsm prompting.
(connect-to-tls-ipv6-nowait): Likewise.
(open-network-stream-tls-wait): New test.
(open-network-stream-tls-nowait): New test.
(open-network-stream-tls): New test.
(open-network-stream-tls-nocert): New test.
(open-gnutls-stream-new-api-default): New test.
(open-gnutls-stream-new-api-wait): New test.
(open-gnutls-stream-old-api-wait): New test.
(open-gnutls-stream-new-api-nowait): New test.
(open-gnutls-stream-old-api-nowait): New test.
(open-gnutls-stream-new-api-errors): New test.
The new tests exercise 'open-network-stream' and the old and new
api of 'open-gnutls-stream'.

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