core: Disable TCP_NOPUSH optimization on OSX. BZ 66019.

core: Disable TCP_NOPUSH optimization on OSX. BZ 66019.

OSX supports TCP_NOPUSH but does not release the data retained (in TCP stack)
when the option is unset. It seems that unsetting it before the last write
does not help either so just disable the optimization for OSX in the core
output filter to avoid uncontrollable transmission delays.

* server/core_filters.c():
  Add the sock_nopush() helper that does nothing on OSX and platforms not
  supporting TCP_NOPUSH or TCP_CORK.

* server/core_filters.c(send_brigade_nonblocking):
  Use sock_nopush() instead of apr_socket_opt_set() for APR_TCP_NOPUSH option.

Merge r1900100 from trunk.

Submitted by: ylavic
Reviewed by: ylavic, rpluem, jorton

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1901201 13f79535-47bb-0310-9956-ffa450edef68