man9: add hz(9) and hardclock(9)

Document aspects of system time keeping. Hz is the nominal rate that we
interrupt the system and is known and the 'tick' period of 1 / hz.
hardclock is the routine that does various bits of timekeeping. stathz
and profhz are documented as historical relics that are deprecated
and replaced by hwpmc.4 and others.

Reviewed by: phk@, mav@ and gnn@ (previous version)
Obtained from: hardclock.9 from NetBSD (with FreeBSD adjustments)
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30802