This implements insecure over-file-socket gRPC API for init with two
first simplest APIs: reboot and shutdown (poweroff).
File socket is mounted only to `osd` service, so it is the only service
which can access init API. Osd forwards reboot/shutdown already
implemented APIs to init which actually executes these.
This enables graceful shutdown/reboot with service shutdown, sync, etc.
Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>
