Add lockmgr(9) probes to the lockstat DTrace provider.

They follow the conventions set by rw and sx lock probes. There is
an additional lockstat:::lockmgr-disown probe.

Update lockstat(1) to report on contention and hold events for
lockmgr locks. Document the new probes in dtrace_lockstat.4, and
deduplicate some of the existing probe descriptions.

Reviewed by: mjg
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21355