Be able to enforce the initialization of a TCP server with a dual stack

[dinosaure]

Highlighted by @hannesm here: #270 (comment), It's not possible to enforce a server to be only initialized on an IPv6 sockaddr. Lwt.setsockopt should fix the problem.

[hannesm]

the issue is a bit more delicate when binding a socket to PF_INET inet6_addr_any:

• Linux opens a dual V4 and V6 socket
• macos & FreeBSD open a V6 only socket

the solution is to call setsockopt fd IPV6_ONLY false in a case where a dual stack socket is requested, and the same with true as last argument if a V6 only socket is requested. AFAICT the information whether a dual or v6-only socket is requested is atm not passed through in conduit, and thus the setsockopt cannot be done.

[vog]

I fully agree to @hannesm, and would like to stress one detail:

• This issue should not be about how to initialize a v6-only service, but how to portably and reliably initialize a v4+v6 service.

[dinosaure]

Ok, I will propose something when #324 will be merged - to avoid any confusion between conduit-lwt/conduit-mirage and to still be homogeneized.