Add AT_EMPTY_PATH for several *at(2) syscalls

It is currently allowed to fchownat(2), fchmodat(2), fchflagsat(2),
utimensat(2), fstatat(2), and linkat(2).

For linkat(2), PRIV_VFS_FHOPEN privilege is required to exercise the flag.
It allows to link any open file.

Requested by: trasz
Tested by: pho, trasz
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D29111