Fix two bugs, one in proactive and the other in reactive flow setup.

1) In _install_proactive_path(), EthAdd expects either 6 raw bytes or a
string (of length 12 or 17); so the fix adds a utility function from
MN to turn an unsigned int into a MAC colon hex string.

2) The flow mod routine barfed for buffer_id = -1, so just don't set it.