rpc: ensure all sockets bind to same port when service is NULL

rpc: ensure all sockets bind to same port when service is NULL

When the service passed to getaddrinfo is NULL the kernel will choose a
free port to bind to. In a dual stack though we will get separate
sockets for IPv4 and IPv6 and we need them to bind to the same port
number. Thus once the kerel has auto-selected a port for the first
socket, we must disable auto-select for subsequent IP sockets and force
reuse of the first port.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>