build: Added Visual Studio 6.0 (VC6) project files

Carrying on from commit 11025613b9 added a more thorough version of
the VC6 project files which are capable of supporting side-by-side
compilation, as well as support for some of the third-party libraries
curl uses.
build: Added Visual Studio .NET (VC7) project files

Carrying on from commit 11025613b9 added VC7 project files which are
capable of supporting side-by-side compilation, as well as support for
some of the third-party libraries curl uses.
build: Corrected libcurl PDB file name for x64 builds in VC8 through VC12
build: Added other VC6 output files to the .gitignore list
http2: Compile with latest nghttp2

commit 6d5f40238028f2d8c (Apr 27) or later nghttp2 is now required