secchan: Only let the controller connection set configuration flags.

secchan: Only let the controller connection set configuration flags.

Each OpenFlow connection to secchan, both controller connection and
management connections, is allowed to set its own miss_send_len and
send_flow_exp configuration, because these are really per-connection
state, not global state.  But management connection should not change
the global state (currently, just the treatment of fragments) when
they do this.