urlapi: parse IPv6 literals without ENABLE_IPV6

[b-spencer]

As per the discussion on the curl-library mailing list, here is a patch to allow URLs with IPv6 literals to be parsed correctly even when ENABLE_IPV6 is not configured, including when the target lacks the AF_INET6 constant (and other IPv6 definitions).

It simply enables the existing curl IPv6 literal parsing capabilities when ENABLE_IPV6 is not defined, and creates a limited-scope definition of AF_INET6 that the URL and IP parsing code (only) use to communicate. It seemed safer to not define AF_INET6 more widely, and it seemed unnecessarily complex to switch curl to define its own constant and use it, so I took a simple pragmatic approach. Feedback on this detail, including the resulting redefinition of AF_INET6 in a three spots, is welcome.

For tests, I simply relaxed the restrictions on the existing tests to now require IPv6 support. I do not know if any CI runs the tests this way (but I did manually).

I don't have a machine that lacks AF_INET6, so I also simulated that case by undefining it.

All tests passed for me on Linux x86_64.

[bagder]

Thanks!