Make the value set for the socket option TCP_DEFER_ACCEPT configurable

Make the value set for the socket option TCP_DEFER_ACCEPT configurable

* include/ap_listen.h:
  - Add prototype for include/ap_listen.heraccept
  - 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

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1927885 13f79535-47bb-0310-9956-ffa450edef68