Stop using eventhandlers for itimers subsystem exec and exit hooks.

[kib]

Summary

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.

[mjg]

General note for changes of the sort is that perhaps given the 'priority' things it would be best to execute these *after* eventhandlers.