bind to ipv6 address

[BChabanne]

It seems that it is not possible for capnp-rpc server to bind to ipv6 addresses.

The error I get when serving such vat config

  let vat_config =Capnp_rpc_unix.Vat_config.create ~secret_key:(`File "/tmp/key") (`TCP ("::1", 7000))

is Failure("Unknown host \"::1\"")

After searching a little bit, I found this related issue ocaml/ocaml#13114
It seems that the function gethostbyname is not supporting ipv6 but getaddrinfo does.

Would it make sense to use getaddrinfo in files unix/network.ml and unix/capnp_rpc_unix.ml ? instead of gethostbyname to support ipv6 ?

If ipv6 is to be supported, Network.Location.of_string would also need to be reworked a bit, because command line parsing would fail with

'--capnp-listen-address': PORT must be an integer

If there are no other big blockers you can think of, I can start to work on a PR to support ipv6