* 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'.
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