Bug#839403: apt: define default pin priority for a source in sources.list directly

[Ansgar <ansgar@debian.org>]

Hi,

I also think this would be nice to have. Adding an extra field to
sources.list is simpler than having to maintain an additional file for
the pinning.

Besides the common use case of adding unstable to a system running
Debain testing, it would also make it easier to add additional suites
to a system running stable to pull in debian-installer-*-netboot-*
packages for multiple releases.

It would also be nice if there were named values as well, like

  Pin-Priority: not-automatic
  Pin-Priority: automatic-upgrades

having similar effect as "NotAutomatic: yes" respectively
"ButAutomaticUpgrades: yes" in Release. (Obivously choosing a nice name
is hard.)

Ansgar