Always check for client-certificates

Always check for client-certificates

* lisp/net/network-stream.el
(network-stream-use-client-certificates): New user option.
(open-network-stream): If 'network-stream-use-client-certificates'
is t, and the user hasn't specified :client-certificate, do
certificate lookups via 'auth-source'.
(network-stream-certificate): Only return key and certificate
files that exist.

* doc/lispref/processes.texi (Network): Document new
client-certificate behavior.

* etc/NEWS: Document 'network-stream-use-client-certificates'.