maclist: remove loc option from mac registration

Implementing this would either require a well-defined edge or a
stateful policy. Not ready for that.
maclist: convert from Policy.t to Pred.t

The maclist implements a MAC address whitelist, which is filter on
packets. No need to use the full-fledged Policy.t interface.
app: use learning from frenetic

This requires using the learning branch of the frenetic repository.
main: enable discovery and whitelist some MACs