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.