mod_proxy, mod_ssl: Handle SSLProxy* directives in <Proxy> sections,

mod_proxy, mod_ssl: Handle SSLProxy* directives in <Proxy> sections,
allowing per backend TLS configuration.
trunk patch: http://svn.apache.org/r1740928
             http://svn.apache.org/r1740960
             http://svn.apache.org/r1740967
             http://svn.apache.org/r1740987
             http://svn.apache.org/r1740998
             http://svn.apache.org/r1742697
             http://svn.apache.org/r1756976
             http://svn.apache.org/r1781313
             http://svn.apache.org/r1812193
2.4.x patch: https://svn.apache.org/repos/asf/httpd/httpd/patches/2.4.x/httpd-2.4.x-r1740928_and_co-v6.patch
+1: ylavic, icing, minfrin

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1824187 13f79535-47bb-0310-9956-ffa450edef68