IP_BIND_ADDRESS_NO_PORT when available
Since Linux 4.2, we can use
Suggested by Willy Tarreau (thanks!).
Since Linux 4.2, we can use `IP_BIND_ADDRESS_NO_PORT` to let the kernel know that we are calling `bind()` only to select the source address without any intention to listen on the socket. That way the source port "will be automatically chosen at `connect()` time, in a way that allows sharing a source port as long as the 4-tuples are unique." Suggested by Willy Tarreau (thanks!).