machinectl: reimplement machinectl's "reboot" verb on top of "kill", and add new verb "poweroff"

There's really no point to send the reboot SIGINT from machinectl
directly, if machined can do that anyway. This saves code, and
makes machinectl network transparent for these verbs. And while we are
at it we can easily add a "poweroff" verb in addition to "reboot". Yay!