Stop using eventhandlers for itimers subsystem exec and exit hooks.
While there, do some minor cleanup for kclocks. They are only
registered from kern_time.c, make registration function static.
Remove event hooks, they are not used by both registered kclocks.
Add some consts.
Perhaps we can stop registering kclocks at all and statically
Reviewed by: mjg
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D27305