Adjust the set of unix gids used for a download server.
[ this is a re-commit of a faulty earlier commit. ] When downloading an image, start the frisbeed process with the minimum set of gids necessary to access the image. This includes the unix gid of the project that the image is in and, optionally, the unix gid of the project subgroup if the image is part of one. Previously, we just use the gid set of the uid of the swapper of the experiment. Not only was this excessive, but it might also not include the gids needed in the case of a "global" image that is not in the world-readable /usr/testbed/images directory.