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
initialize them.

Reviewed by: mjg
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D27305