server: support dual-stack server socket

[ottigeda]

According to RFC 3493 the socket option IPV6_V6ONLY
should be off by default, see
https://tools.ietf.org/html/rfc3493#page-22 (chapter 5.3).

However this does not seem to be the case on all systems.
For instance on any Windows OS, the option is on by default.

Therefore clear this option in order to allow
an server socket which can support IPv6 and IPv4 at the same time.

Would this change be acceptable to you ?
Or should I make it configurable, using an #define or an param on the server class ?

[ottigeda]

As it seems to me, the test on "Travis CI" can connect to "127.0.0.1" but not to "::1".
Do you know if that is a problem of my code, or is this rather related to the system the test is running on ? Do you know what could be the difference ?

[yhirose]

@ottigeda, sorry for the late reply. I actually don't know why. But ipv6 support seems to be disabled on the TravisCI. https://github.com/njh/travis-ipv6-test

If you think TravisCI is not trustworthy as for IP network, I don't mind retiring it since I can now test Mac OS and Linux on GitHub Action.

[ottigeda]

I think it should really be you to decide that.
However in my opinion supporting IPv6 becomes more important, since the IPv4 addresses are in short supply these days.
It is for sure still an long way to go for IPv6, but if the infrastructure does not support it,
it is difficult to make progress in applications to make them support it.

[yhirose]

@ottigeda, I removed TravisCI test. Thank you for your contribution!