this saves us from the small chance that a client accepted before reconf uses the flags from a listener post reconf.

drop connections in the handshake phase: the listener from where they've been accepted is now being replaced and its id is no more meaningful. Clients for which we have already sent an auth request are kept.

it's not so useful here and not (directly) used anywhere else.

and don't forget to free the current config!

delete useless functions such as merge_config and config_clear while here.

Passing a NULL pointer to memcpy is UB no matter if len is 0 (commit by claudio@, backporting for kamid.)