hostip: Make Curl_ipv6works function independent of getaddrinfo

[jay]

• Do not assume IPv6 is not working when getaddrinfo is not present.

The check to see if IPv6 actually works is now independent of whether
there is any resolver that can potentially resolve a hostname to IPv6.

Prior to this change if getaddrinfo() was not found at compile time then
Curl_ipv6works() would be defined as a macro that returns FALSE.

When getaddrinfo is not found then libcurl is built with CURLRES_IPV4
defined instead of CURLRES_IPV6, meaning that it cannot do IPv6 lookups
in the traditional way. With this commit if libcurl is built with IPv6
support (ENABLE_IPV6) but without getaddrinfo (CURLRES_IPV6), and the
IPv6 stack is actually working, then it is possible for libcurl to
resolve IPv6 addresses by using DoH.

Ref: #7483 (comment)

Closes #xxxx

[jay]

It may be possible to improve this for c-ares IPv6 when getaddrinfo is not present in curl, by using CURLRES_IPV6 for c-ares, but it seems unusual that getaddrinfo would be in one and not the other.

curl/lib/curl_setup.h

Lines 565 to 577 in bfbde88

	#if defined(ENABLE_IPV6) && defined(HAVE_GETADDRINFO)
	# define CURLRES_IPV6
	#else
	# define CURLRES_IPV4
	#endif
	#ifdef USE_ARES
	# define CURLRES_ASYNCH
	# define CURLRES_ARES
	/* now undef the stock libc functions just to avoid them being used */
	# undef HAVE_GETADDRINFO
	# undef HAVE_FREEADDRINFO
	# undef HAVE_GETHOSTBYNAME

[MarcelRaad]

👍

[bagder]

I wouldn't have bothered since such platforms are virtually not used, but since you've already written the code...