datapaths: Rework port add/remove

This replaces the take_port_down/bring_port_up APIs with add_port and
delete_port.  These actually remove the ports instead of adding them
to a set of ports to ignore (this is closer to some older behavior).

Also:
* slightly refactor port generation
* normalizes some log messages related to packet drops
* makes it so the port_down config bit will actually drop packets
* adjusts tests
datapaths: Rename process_packet()

Renamed to rx_packet() to be consistent.