mod_ssl: Add SSLProxyMachineCertificateChainFile directive uses openssl

to construct a chain for each proxy cert. When a remote server requests
a client certificate that is NOT the direct issuer of any available client
certificate, the chain for that certificate will be used to trace it to a
known CA and that client certificate will be used. 

Submitted by: druggeri
Reviewed by: kbrand, rpluem
Backports: 1160863,1162103,1170833,1172010,1175946,1242089



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1374374 13f79535-47bb-0310-9956-ffa450edef68