mod_ssl: Catch missing, mismatched or encrypted client cert/key pairs

with SSLProxyMachineCertificateFile/Path directives. 
(check at startup, to prevent segfaults at proxy request time)

PR: 52212, 54698
Backports: r1374214, r1374216, r1375445, r1467593
Submitted by: Keith Burdis <keith burdis.org>, Joe Orton, Kaspar Brand
Reviewed by: kbrand, minfrin, wrowe



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1497096 13f79535-47bb-0310-9956-ffa450edef68