API_PROXY: CPLIsFilenameRelative() doesn't work well with /vsisparse/ , /vsizip/, etc... so always pass the current working dir to the server; GetFileList(): If server is Windows and client is Unix, then replace backslahes by slashes

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@25632 f0d54148-0727-0410-94bb-9a71ac55c965