New 'maint' facet for Debtags

[Florian Weimer]

* Enrico Zini:

> Below is an example vocabulary for it, with annotations on how to
> autogenerate the tag information.  I'd like to run some discussion on it
> for a week or so, then proceed to implementation.

Something that indicates security support by upstream, the maintainer
and the security team would be helpful, but it could pretty complex
(we'd need to differentiate between stable and unstable/testing, too,
I guess).