Sandbox configuration key to specify uid/gid for sandbox

This is a replacement for !301 (closed) since it's almost a complete rewrite. Please see the discussion on that. This MR adds a top-level key for project.conf and element files which can specify build-uid and build-gid, which will be the uid and gid used by the sandbox, if the sandbox is capable of changing that. The default remains the same, using uid/gid 0/0 to build.