by making it accept some open(2) flags. More precisely, only
O_CLOEXEC is supported, the flag is translated into the KQUEUE_CLOEXEC flag
for kqueuex(2), and O_NONBLOCK is silently ignored.
Reported and tested by: vishwin
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D39377
