tcp: socket option to set TCP fast open key

tcp: socket option to set TCP fast open key

New socket option TCP_FASTOPEN_KEY to allow different keys per
listener.  The listener by default uses the global key until the
socket option is set.  The key is a 16 bytes long binary data. This
option has no effect on regular non-listener TCP sockets.

Signed-off-by: Yuchung Cheng <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Reviewed-by: Christoph Paasch <[email protected]>
Signed-off-by: David S. Miller <[email protected]>