Merge r1927885, r1927916, r1928022 from trunk:

Merge r1927885, r1927916, r1928022 from trunk:

Make the value set for the socket option TCP_DEFER_ACCEPT configurable

* include/ap_listen.h:
  - Add prototype for ap_set_listentcpdeferaccept
  - Wire in new directive ListenTCPDeferAccept

* include/mpm_common.h:
  Define the previous static value as default value via DEFAULT_TCP_DEFER_ACCEPT

* server/listen.c:
  - Add static int ap_listentcpdeferaccept
  - ap_apply_accept_filter: Use value of ap_listenbacklog for setting TCP_DEFER_ACCEPT
  - ap_listen_pre_config: Set default value
  - Add ap_set_listentcpdeferaccept

* Follow up to r1927885: Use correct configuration variable. Thanks to @ylavic
  for finding this.

* Follow up to r1927885: Changelog entry and documentation

Reviewed by: rpluem, jorton, covener

Github: closes #555

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1929574 13f79535-47bb-0310-9956-ffa450edef68