net: add server support.

Creating a server for IPv4 and IPv6 has similar issues to clients,
with some novel twists.  Slightly different arguments need to be given
to getaddrinfo(), but worse, some platforms (Linux without
/proc/sys/net/ipv6/bindv6only set) automatically bind IPv6 sockets to IPv4
ports as well.

Thus we need a function which can bind (and listen) to one or two fds.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>