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.
As these files don't need to contain references to the source files,
although typically do, added basic files which only include three
filters and don't require the project file generator to be modified.
These files allow the source code to be viewed in the Solution Explorer
in versions of Visual Studio from 2010 onwards in the same manner as
previous versions did rather than one large view of files.
