Add option to order device startup in ifup

[noc0lour]

This allows to safely configure bonds, VLAN and bridges on top without having to fiddle with names.
This can be done by specifying order in any device stanza which will then evaluate to the
smallest order set in any stanza for that device. Parameter order will evaluate to 99 if it is not set
for that device.

The resulting files are then named device-$order_$devicename.

Would be great to have that functionality upstream.

On top this adds a default vars/ file for debian-9, which is of course optional.

[arBmind]

hi @noc0lour,
Thank you for your pull request. It looks good.

One question: Why device-${order}_${devicename} and not ${order}_device-${devicename}? - I do not really remember why we added the device_ prefix.

[noc0lour]

No specific reason, I can change that. Is ifupdown/networking expecting a special name format in interfaces.d with device prefixed?

[arBmind]

@noc0lour - as far as I interpret the http://manpages.ubuntu.com/manpages/xenial/man5/interfaces.5.html - it includes all files of a directory.