Add a kern.lockf sysctl to list the advisory byte-range locks on a
specific vnode, described by its pid and file descriptor arguments.
A new struct xlockf is added, which contains fields from struct
flock with the addition of xl_id, the address of struct file for
the F_FLOCK locks which have pids of -1.
lf_iteratelocks_sysid() and lf_iteratelocks_vnode() are changed to
use struct xlockf, and to set the real lock type instead of
F_UNLCK so we can report on it, while its other callers overwrite
that type with F_UNLCK as needed.