Fix the inheritance of the non-blocking option across apr_socket_accept()
on FreeBSD 10 which was introduced with APR 1.5.0 through an unlikely
mechanism:

* FreeBSD 10 introduced accept4().  APR uses accept4() where it can find it.
  accept4() on Linux and FreeBSD 10 both have a SOCK_NONBLOCK flag, but on 
  FreeBSD 10 the SOCK_NONBLOCK is the sole determiner of whether or not the
  connected socket is non-blocking.
* clang is normally used on FreeBSD 10.
* APR's configure-time check for inherited O_NONBLOCK didn't work with clang,
  so initially the lack of inheritance across accept4() wasn't a problem.
* APR 1.5.0 allowed the configure-time check to work with clang, exposing
  the bad expectation about accept4() matching the accept() behavior.

With FreeBSD accept4() (avail in 10+), O_NONBLOCK is not inherited
(unlike Linux).  Mimic the accept() behavior here in a way that
may help other platforms as well.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1551659 13f79535-47bb-0310-9956-ffa450edef68