The Visual Studio builds didn't use IPv6. Add it to all projects since
Visual Studio 2008, which is verified to build via AppVeyor.
Closes https://github.com/curl/curl/pull/3137
After squashing most of our compiler warnings, up'ed the default
warning level from 3 to 4 in order to increase the likelyhood of
catching future warnings.
Updated the makefiles and Visual Studio project files to support moving
the authentication code to the new lib/vauth directory that was started
in commit 0d04e859e1.
