gethostbyname() in validating URLs does not work for IPv6

[frankieandshadow]

Summary:

wp_http_validate_url() rejects valid URLs when the hostname resolves to IPv6-only addresses, because gethostbyname() only supports IPv4. I discovered this with a custom plugin repository in a IPv6 only network (so the DNS only has a AAAA record when it validates the URL provided to fetch the plugin package).

Expected behavior:
URLs with hostnames that resolve to IPv6 addresses should be validated successfully.

Actual behavior:
When a hostname resolves only to IPv6, gethostbyname() returns the hostname unchanged, which the function treats as a DNS lookup failure and rejects the URL.

How to reproduce:
Create a hostname that resolves only to IPv6 (AAAA record, no A record)
Try to use that URL in any WordPress HTTP API call with default settings
The URL will be rejected with "A valid URL was not provided"

Suggested fix:
Replace gethostbyname() with dns_get_record() or similar that supports both IPv4 and IPv6 (or use getaddrinfo() via a PHP extension.

Workaround:
Set reject_unsafe_urls to false in http_request_args filter for affected URLs.

This PR implements a function for getting the IP for a hostname, similar to gethostbyname but also working for IPv6. So, hosts like ipv6.google.com would be valid URLs and the hostname would be correctly obtained.

Trac ticket: https://core.trac.wordpress.org/ticket/64170