Modern versions of D-Bus have a BecomeMonitor method which allows a
sufficiently-privileged user (ie root) to capture all messages on the
system bus. Modern versions of dbus-monitor use this method, and support
pcap output natively.

This allows us to use pkexec to escalate just the dbus-monitor process,
and so add a way to monitor the system bus from within the UI.  We can
also use Flatpak's HostCommand method (via `flatpak-spawn --host`) to do
the same from within the Flatpak sandbox. It's not much extra effort to
support monitoring an arbitrary bus by address, so that's wired up too.

Bustle itself still connects to the bus you're monitoring to dump all
current names, so will still require full session and system bus access.
If this fails, it's not considered fatal.

We assume that dbus-monitor is present on the host, and recent enough to
support --pcap. We'll see if this assumption is true in practice!