PackageKit now has native FreeBSD pkg backend

Good news everyone [who uses FreeBSD as a desktop system]!

The latest release of PackageKit now has a more or less functional pkg
backend and is committed to our ports tree.

PackageKit is a small D-Bus daemon program that serves as a backend
for "application store" type of apps - most notably Plasma Discover
and Gnome Software Center. Having a working PackageKit backend means
that you can now use them on FreeBSD to manage software (without root
privileges [*]). Plasma Discover is already switched to using
PackageKit, so you'll get it working out of the box once you update
your ports/packages.

Note that there is still another component required to fully utilize
application store apps - AppStream. The work on this front is being
done too, but for now you won't be able to search and install packages
- only update them.

You can also use PackageKit from the command line via the "pkcon"
utility. There is not much point in this unless we'll get another
backend for PackageKit. For now it is a substitute for the regular
"pkg" command.

If you observe any crashes or bugs in PackageKit please let me know by
opening an issue upstream
https://github.com/PackageKit/PackageKit/issues
If you're interested in contributing, there is a lot of work to do too!

I'd like to thank bapt@ for all the support he provided on getting
required pieces into libpkg and fixing the breakages introduced by my
PRs.

The work was sponsored by Serenity Cybersecurity LLC and we have even
more to unleash on you in future. Hold tight!

Cheers.

[*] Instead, polkit is being used to grant the user an authorization
for specific actions.