Add Unicode and IDN support

[xdqi]

This pull request introduces support compiling with UNICODE.
You can also use IDN domain names (not punycode ones) after the pull request.

[White-Tiger]

I'm sorry that you've done this kind of work again... I had it already done myself, it's just not published yet.
Though, I also did consider to use IdnToAscii() but dropped it in the end. I don't really remember the reasons though. How's XP support for that one?

[xdqi]

On some systems it cannot resolve hostname without translating it with IdnToAscii.
XP do not ship IDN related DLL by default so I load it with LoadLibrary and GetProcAddress.

[White-Tiger]

Well... I do know that IDN support is quite important.. that's why I've implemented it back in 2014 (or rather my first implementation which wasn't complete... as there's no freely available library that supports UTF-16 and I don't like to convert it to UTF-32 first...)
But that's the reason why it should also work with XP (probably Win2k) which IdnToAscii won't.

I guess “On some systems [...]” meant people who use Unicode domains? Like russians, germans and that like?