dbus: move systemd-hooks for dbus.socket to 'dbus-common'
    
    The systemd_{post,postun,preun,..} hooks should be called in the package
    that actually provides the given files. There is no harm in calling
    these in the dbus-daemon package, but preferably we don't. Move this to
    'dbus-common' for dbus.socket, since dbus.socket is provided by it. This
    makes sure the unit is properly enabled/disabled even without the
    dbus-daemon package installed.
    
    Note that it is safe to enable/disable dbus.socket even without a
    dbus.service around. systemd can deal with this just fine, and only ever
    actually pulls in the socket if an activatable service is around.
    
    Signed-off-by: David Herrmann <dh.herrmann@gmail.com>