Update cpucontrol(8).

Mention new -n flag.
Remove optional -h from the operation list lines, -h would cause the
utility to exit without performing the action.
Explain the default path behavior, list default path.
Correct example of update performed from the non-default path,
it needs -n and the trailing slash is redundand.
Remove useless BUGS section.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week