W32 GFileInfo improvements

This is a followup for issue #1452 (closed).

Now that junction points and mountpoints are considered "symlinks" by GFileInfo, we probably need a way to tell them apart from any real symlinks. The new G_FILE_ATTRIBUTE_ID_DOS_IS_MOUNTPOINT helps with that. G_FILE_ATTRIBUTE_DOS_REPARSE_POINT_TAG is to provide some info about other kinds of reparse points that glib doesn't know how to handle.

Also, another commit here prevents glib from stripping extended path prefixes from absolute paths that do not contain a drive letter and a colon. This way extended paths like \\?\Volume{GUID}\ will remain unmolsted, which is more useful for the user than getting Volume{GUID}\ and then trying to figure out whether it's a relative directory or a volume name.