Fix detection of O_NONBLOCK inheritance.

The original test failed occasionally on a busy
FreeBSD server when accept() returned EAGAIN.

Backport of r1449568 from trunk.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/branches/1.5.x@1449569 13f79535-47bb-0310-9956-ffa450edef68