bus-polkit: add support for authenticating varlink peers via polkit

bus-polkit: add support for authenticating varlink peers via polkit

This extends our current polkit logic, so that we can in a very similar
fashion as we already can authenticate dbus peers authenticate varlink
connection peers.

polkit natively speaks dbus and can authentication dbus peers. To get
the same level of support for varlink we'll use authentication by
pidfd+uid. This requires polkit v124, and if that's not available it
will fallback to authorizing root only as before.

Co-authored-by: Luca Boccassi <bluca@debian.org>