systemd immediately kills ifplugd

[H NAKAMURA]

Ifplugd will be immediately killed by systemd so the system can no longer detect ethernet's link status and re-run dhclient.

Ver. of ifplugd: 0.28-19ubuntu1

Environment:
* OS: Ubuntu 16.04.02 Server Ed. (Amd64)
* Network-Manager is NOT used, ifplugd+dhclient is used instead

The actual behavior:
* some ethernet device detected (like USB-ethernet device is plugged-in)
* Systemd-udev reads /lib/udev/rules.d/40-ifplugd.rules, probably
* systemd-udev runs /lib/udev/ifplugd.agent
* ifplugd.agent runs ifplugd
* ifplugd forks its deamon
* ifplugd.agent exits
* systemd-udev thinks the udev task (ifplugd.agent) is done, and probably kill all forked processes including ifplugd daemon

Expected behavior:
* ifplugd should continue running to monitor the network device.
* It worked on Ubuntu 14.04, but not on 16.04

Other:
# cat /etc/default/ifplugd
INTERFACES=""
HOTPLUG_INTERFACES="eth1 eth2 eth3 usb0 usb1 usb2 usb3"
ARGS="-q -f -u0 -d10 -w -I"
SUSPEND_ACTION="stop"