MINOR: tcp: add new bind option "tcp-ss" to instruct the kernel to save the SYN

MINOR: tcp: add new bind option "tcp-ss" to instruct the kernel to save the SYN

This option enables TCP_SAVE_SYN on the listening socket, which will
cause the kernel to try to save a copy of the SYN packet header (L2,
IP and TCP are supported). This can permit to check the source MAC
address of a client, or find certain TCP options such as a source
address encapsulated using RFC7974. It could also be used as an
alternate approach to retrieving the source and destination addresses
and ports. For now setting the option is enabled, but sample fetch
functions and converters will be needed to extract info.