netplug: look for init script configurations in /etc/default/

netplug: look for init script configurations in /etc/default/

We are working to make all sysvinit scripts conformant to a pattern and
/etc/default/ seems to be a good choice, since 34 packages already get
optional configurations from files at that directory.

netplug still installs an init script at /etc/rc.d/init.d/.  This will
be fixed in a future patch that will refactor the init scripts.

Signed-off-by: Carlos Santos <casantos@datacom.com.br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>