open(2): Remove O_BENEATH and AT_BENEATH
with the reasoning that the flags did not worked properly, and were not
shipped in a release.
O_RESOLVE_BENEATH is kept as useful.
Reviewed by:	markj
Tested by:	arichardson, pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D28907