VS projects: use Unicode for VC10+

All Windows APIs have been natively UTF-16 since Windows 2000 and the
non-Unicode variants are just wrappers around them. Only Windows 9x
doesn't understand Unicode without the UnicoWS DLL. As later Visual
Studio versions cannot target Windows 9x anyway, using the ANSI API
doesn't really have any benefit there.

This avoids issues like KNOWN_BUGS 6.5.

Ref: https://github.com/curl/curl/issues/2120
Closes https://github.com/curl/curl/pull/3720