build: Use dbus-run-session

DBus 1.8 gained a utility called dbus-run-session, that will run another
program inside a new session bus and then shut down the session bus
afterwards. This can replace the hand-rolled run-with-dbus script.

Even though we started a fake system bus with run-with-dbus, I don't
believe it was used. We also generated extra configuration files with
which to start the buses, but I don't believe those were necessary
either.

https://bugzilla.gnome.org/show_bug.cgi?id=771745