sbus: add utility function to simplify message and reply handling
    
    This patch adds the ability to hook DBusMessage to a talloc context
    to remove the need of calling dbus_message_unref(). It also provides
    an automatical way to detect error in a reply so the caller does
    not need to parse it manually and the whole code around DBusError
    can be avoided.
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>