dnsdist: Use `IP_BIND_ADDRESS_NO_PORT` when available

dnsdist: Use `IP_BIND_ADDRESS_NO_PORT` when available

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!).