ab: Add client certificate support.

trunk: http://svn.apache.org/r1841784
2.4.x: svn merge -c r1841784 ^/httpd/httpd/trunk .
+1: minfrin, jim, ylavic


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1843412 13f79535-47bb-0310-9956-ffa450edef68