Merge r1496429 from trunk:

core: Support the SINGLE_LISTEN_UNSERIALIZED_ACCEPT optimization
on Linux kernel versions 3.x and above.

PR: 55121
Submitted by: Bradley Heilbrun <apache heilbrun.org>
Reviewed by: trawick

Submitted by: trawick
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1496916 13f79535-47bb-0310-9956-ffa450edef68