Use gvplugin_list instead of gvPluginList

Use gvplugin_list instead of gvPluginList
Make graphviz unix only for several reasons:
1) Win32 graphviz doesn't have the gvplugin_list() exported and we don't have a word to check if an ffi call exists before attempting to call it so we are unable to find a two-state solution with both gvplugin_list and gvPluginList
2) Compiling the Graphviz Project on Windows is extremely broken under Cygwin, MinGW and Visual Studio 2008 and fails to compile without major changes and still has disabled projects and thousands of warnings/errors. Th HOWTO on their website is outdated on all counts and the maintainer has not replied to my email asking for help. I do not have time to be a Graphviz maintainer myself.
3) When I did manage to compile it on Windows, none of the engines are avaialable to the dlls we load so the compiled Graphviz has no features except symbol exports. The same is true for the binary package--we do not have access to the
4) There is no win64 binary of Graphviz on their website and compiling it would likely result in an unusable set of dlls as per above