TCP implementation not sending an appropriate MSS option

Michael Welzl pointed out that our TCP implementation does not send the MSS option as expected, which has become a MUST requirement in the TCP RFC 9293.

As a result, most programs will use the (obsolete) 536 byte segment size unless the user configures a manual override.

The default for IPv6 when MSS is not received should also be changed to 1220 bytes.