Enable and use WinIDN and AppleIDN by default on these platforms,
if they can be used.

Compatibility warning: This change can make default builds incompatible
with Windows XP by default. WinIDN requires Normaliz.dll. Windows XP
isn't shipping with it and the necessary extra package is no longer
offered by Microsoft. Thus, for Windows XP compatibility, we recommend
building with -DUSE_WIN32_IDN=OFF.

Discussion: #13565
Closes #13947

Cleaner diff without whitespace changes: https://github.com/curl/curl/pull/13947/files?w=1

There are a few ways to resolve compatibility with existing builds
and also to handle interactions with the USE_LIBIDN2 option.
This is an initial attempt.

TODO:

• rethink defaulting logic to honor an explicitly selected libidn2?
• is WinIDN useful without Unicode? [YES] Enable it only in Unicode builds by default? (Also needs reshuffling options in Azure and GHA/Windows)
  Downside: it makes the defaulting logic difficult to predict.
• rebase on configure: fix WinIDN builds targeting old Windows #14680. (delete explicit WinIDN options added to GHA/Windows/cross.)
• rebase on GHA/macos: drop options no longer necessary #14693.
• add jobs that disable AppleIDN/WinIDN for both autotools and cmake.
• configure: enable WinIDN by default for HAVE_WIN32_WINNT >= 0x0600, as cmake.
• review and fixup AppleIDN/WinIDN and libidn2 interactions in autotools and cmake.
• tackle AppleIDN.
• cmake: detect WinXP target using HAVE_WIN32_WINNT and adjust default USE_WIN32_IDN setting accordingly?
• review and update CI jobs where/if necessary.
• fix autotools logic to only make WinIDN the default when targeting native Windows.
• figure out how to make IDN tests work on Windows. [SKIP]
• split off VS2008 log dump on failure in AppVeyor into separate PR. → appveyor: dump build logs on failure in VS2008 jobs #13957