openat(2): add O_EMPTY_PATH

It reopens the passed file descriptor, checking the file backing vnode'
current access rights against open mode. In particular, this flag allows
to convert file descriptor opened with O_PATH, into operable file
descriptor, assuming permissions allow that.

Reviewed by: markj
Tested by: Andrew Walker <awalker@ixsystems.com>
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D30148