parseConf() only recognized "port N name" in conf_cmds, so "port N <speed>"
lines were treated as unknown commands and dropped on save - configured port
speeds never persisted.
Add the speed command (incl. optional half/full duplex suffix) to conf_cmds,
add a "port N" entry to conf_overwrite so re-saving a speed replaces the
previous value, and guard the per-port name entry so changing a port's speed
no longer wipes its configured name.