Use systemd service manager notification

Use systemd service manager notification

Notify systemd service manager when our initialization sequence
completed. This helps ordering services as dependencies can rely on vpn
being available.

v2: Add curly brackets (and indention) to block the else-part, msg()
    call was non-conditional before.

v3: Move systemd header include from init.h to init.c.

Signed-off-by: Christian Hesse <mail@eworm.de>
Tested-By: Richard Bonhomme <fragmentux@gmail.com>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20161201213104.5667-1-list@eworm.de>
URL: http://www.mail-archive.com/search?l=mid&q=20161201213104.5667-1-list@eworm.de
Signed-off-by: David Sommerseth <davids@openvpn.net>