Add dataplane-flowstat-plugin

[dungmv56]

A plugin for dataplane allow viewing flow statistics.

License: some files were modified from DANOS which licensed by LGPL/GPL/BSD, others file was licensed by Apache 2.0. For more detail see debian/copyright for list files.

[nickbroon]

These together form the full feature:
danos/vyatta-dataplane#8
danos/danos-service-flowstat#1

[JammyStuff]

I believe there's a license incompatibility here. flowstat.h is being licensed under Apache 2.0, which has patent clauses. This would be an additional restriction over the LGPL 2.1 licensed code being used in flowstat.c.

In general, my suggestion would be to stick to the assortment of licenses that were being used in the seed code. Those are GPL 2.0, LGPL 2.1, MPL 2.0, and BSD 3-clause (for YANG). We probably need to post a page on the wiki outlining what guidelines we used when selecting licenses for pieces of code. I know that this conflicts with what @nickbroon said in danos/vyatta-dataplane#8, but I think this is better advice in general due to the incompatibility of Apache 2.0 with the majority of the rest of the DANOS code.

In this particular case, I believe the best option would be to relicense everything in this repo to LGPL 2.1. Another possible option would be to use a license of Apache-2.0 OR LGPL-2.1-only, although before taking that as advice, I'd want others to weigh in on if that would work or not. I'm also not sure as to the advantages of doing that, as if Apache 2.0 was selected because of the patent provisions, an OR would allow anyone to choose to take the code under LGPL 2.1 instead and not have those patent provisions apply.

[JammyStuff]

License incompatibility needs to be resolved.

[nickbroon]

Completely agree, the licenses need to be compatible with each other. So as mention flowstat.c including flowstat.h cannot use lgpl and apache2 as they are incompatible.
For completely new work, then guidance on suitable licenses to use is probably needed. GPL 2.0, LGPL 2.1, MPL 2.0, and BSD 3-clause (for YANG), as used across most of the repositories, seems appropriate to me.