openflow: Reset port info on features reply

Port info is initially gotten from the features reply that comes
as part of connection establishment.  After that, we independently
track changes as seen from individual port updates.  This
(untested!) patch clears those updates when we see another
features reply, as the features reply should be ground truth.
libopenflow: Convenience methods on ofp_phy_port

This adds some helpers for turning on and off bits in ofp_phy_ports
so that we need less bit-flipping logic scattered elsewhere.
spanning_tree: Switch features after port changes

After we update ports, we request switch features so that the
Connection's port info will be refreshed.
dhcpd: Improve packet_in handler logic and logging