MEDIUM: sock: use protocol when creating socket

MEDIUM: sock: use protocol when creating socket

Use the protocol configured for a connection when creating the socket,
instead of always using 0.

This change is needed to allow new protocol to be used when creating
the sockets, such as MPTCP. Note however that this patch won't change
anything for now, as the only other value that proto->sock_prot could
hold is IPPROTO_TCP, which has the same behavior as 0 when passed to
socket.