Fixes <http://bugs.gnu.org/15890>. * nix/libstore/globals.hh (Settings) Add clientUid and clientGid. * nix/nix-daemon/nix-daemon.cc (daemonLoop] Store UID and GID of the caller in settings. * nix/libstore/build.cc (_chown): New function. (DerivationGoal::deleteTmpDir): Use it, change ownership of build directory if it is kept and the new owner is not root.