The reason is that `add()` is too confusing when working
with ports:

    #
    # this code creates an interface, not adds
    # an existing one as a port!
    #
    ndb.interfaces['br0'].ports.add(…)

    #
    # so now it looks like that:
    #
    ndb.interfaces['br0'].ports.create(…)
    #
    # which is more correct