daemon-util is used throughout the code for starting a daemon whenever
this is needed (e.g. during cluster bootstrapping or master failover).

In order not to confuse systemd and its service supervision code,
daemon-util needs to cooperate with it. Thus, we use systemctl for all
operations if both of these conditions hold:

 - systemd is running as PID 1, checked via the existence of
   /run/systemd/system, as per sd_booted(3).
 - systemd has the ganeti.target unit loaded, indicating that the
   relevant systemd unit files have been installed.

Signed-off-by: Apollon Oikonomopoulos <apoikos@gmail.com>
Reviewed-by: Jose A. Lopes <jabolopes@google.com>