Implement procstat(1) advlocks command

to display list of the advisory file locks in the system.

Example output
$ procstat advlock
RW TYPE PID SYSID FSID RDEV INO START LEN PATH
RO FCNTL 5836 0 0x878700ff02 0xffffffffffffffff 57 0 0 /tmp/2
RW FLOCK -1 0 0x878700ff02 0xffffffffffffffff 13 0 0 /tmp/1

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D34834