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

with SSLProxyMachineCertificateFile/Path directives. PR 52212, PR 54698.
(check at startup, to prevent segfaults at proxy request time)

trunk patches: https://svn.apache.org/r1374214
               https://svn.apache.org/r1374216
               https://svn.apache.org/r1375445
               https://svn.apache.org/r1467593
2.4.x patch: https://people.apache.org/~kbrand/PR52212_54698_2.4.x.patch

Submitted by: kbrand
Reviewed by: jorton, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1476685 13f79535-47bb-0310-9956-ffa450edef68