Makes the criu RPC socket socket-activated with
systemd [1], meaning that systemd will create and listen to
the UNIX socket /var/run/criu-srvice.socket
on behalf of criu until a connection comes in, when it will
then pass control of the socket, along with the first connection
over to a newly spawned criu daemon.

This is similar to inetd, but criu stays around after getting
started, listening itsself on the socket.

[1] http://0pointer.de/blog/projects/socket-activation.html

v2: stripped down sd-daemon.[ch]
    moved units to scripts/sd

v3: stripped down further by removing unneeded includes
Signed-off-by: Shawn Landden <shawn@churchofgit.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>