Allow absolute paths for O_BENEATH.

The path must have a tail which does not escape starting/topping
directory. The documentation will come shortly, see the man pages
commit message for the reason of separate commit.

Reviewed by: jilles (previous version)
Discussed with: emaste
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D17714