There is no problem in practice because this server nonce is generated by
WeeChat itself and encoded to base64, so it never contains any UTF-8 char on
more than one byte.
This introduces a new command called "handshake" in the weechat relay protocol.
It should be sent by the client before the "init" command, to negotiate the way
to authenticate with a password.
3 new options are added:
* relay.network.auth_password
* relay.network.hash_iterations
* relay.network.nonce_size