Merge r1661258 from trunk:

mod_ssl: Add the SSL_CLIENT_CERT_RFC4523_CEA variable, which provides
a combination of certificate serialNumber and issuer as defined by
CertificateExactMatch in RFC4523.

Submitted by: minfrin
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1664363 13f79535-47bb-0310-9956-ffa450edef68