MPMs: Support SO_REUSEPORT to create multiple duplicated listener

records for scalability.

Submitted by: Yingqi Lu <yingqi.lu@intel.com>, Jeff Trawick,
              Jim Jagielski, Yann Ylavic

Reviewed by: ylavic, jim, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1705492 13f79535-47bb-0310-9956-ffa450edef68