core: Support the SINGLE_LISTEN_UNSERIALIZED_ACCEPT optimization

on Linux kernel versions 3.x and above.

PR: 55121
Backports: r1496429
Submitted by: Bradley Heilbrun <apache heilbrun.org>
Reviewed by: trawick, wrowe, rjung



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1497443 13f79535-47bb-0310-9956-ffa450edef68