launch-environment: start dbus-daemon inside the user session
    
    We currently start dbus-daemon separately from the launch
    environment PAM session.  This means all things activated
    by that bus get started in the wrong logind session and are
    missing important environment variables (such as XDG_RUNTIME_DIR).
    
    This commit deletes a bunch of code for managing the dbus session
    separately, and instead just makes it part of the session command
    (using dbus-launch as a wrapper around the session).
    
    Because we no longer have the specific PID of the bus daemon,
    we now stop the launch environment by killing the whole process
    group in one go.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=684474