mod_ssl: Correctly merge configurations that have client certificates set

mod_ssl: Correctly merge configurations that have client certificates set
by SSLProxyMachineCertificate{File|Path}.

The certificates and keys loaded during configuration time got lost during
runtime if e.g. SSLProxyMachineCertificate{File|Path} was set on virtual host
level and there was an SSL directive at directory level, e.g. SSLRequire.

This fixes a regression likely introduced in r1740928 (backported in r1824187).

Backport of r1844002 from trunk.

Submitted by: rjung
Reviewed by: rjung, rpluem, jorton

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1844226 13f79535-47bb-0310-9956-ffa450edef68