netplug: prevent starting multiple instances of netplugd

netplug: prevent starting multiple instances of netplugd

Executing "/etc/init.d/S29netplug start" multiple times resulted in
multiple instances of netplugd.

Pass "-p /var/run/netplug.pid" to netplugd, so it creates the PID file
that start-stop-daemon needs to know that netplugd is already running.
Also use the pid file to stop netplugd, instead of the daemon name.

Fixes https://bugs.busybox.net/show_bug.cgi?id=10661

Reported-by: Joachim Krueger <mail2k@web.de>
Signed-off-by: Carlos Santos <casantos@datacom.com.br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>