win32: read system rcpath from registry Using the module name was not always helpful. It breaks down when Mercurial is installed as source and when the Mercurial libs are used by external applications. This patch allows Mercurial installers to store the system wide rcpath in the registry, where it can always be found. HGRCPATH is a poor option for storing the system wide rcpath, since it overrides both the system and user rcpaths.