id: fail when getuid, getgid, etc. fail, e.g., on GNU/Hurd

POSIX-conforming getuid, geteuid, etc. functions cannot fail,
but on GNU/Hurd systems and some others, they may.
* src/id.c (main) [__GNU__]: Detect and diagnose any such failure.
* tests/id/gnu-zero-uids: New file.
* tests/Makefile.am (TESTS): Add it to the list.
* tests/init.cfg (require_gnu_): New function.

tests/Makefile.am

@@ -399,6 +399,7 @@ TESTS =						\
   du/slink					\
   du/trailing-slash				\
   du/two-args					\
+  id/gnu-zero-uids				\
   id/no-context					\
   install/basic-1				\
   install/create-leading			\

tests/id/gnu-zero-uids

@@ -0,0 +1,29 @@
+#!/bin/sh
+# On GNU, `id' must fail for processes with zero UIDs.
+
+# Copyright (C) 2011 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+print_ver_ id
+
+require_gnu_
+
+sush - true || skip_ "the \`sush' command does not work"
+
+# Run `id' with zero UIDs.  It should exit with a non-zero status.
+sush - id > out && fail=1
+
+Exit $fail

tests/init.cfg

@@ -499,4 +499,11 @@ print_ver_()
   fi
 }
 
+# Are we running on GNU/Hurd?
+require_gnu_()
+{
+  test "$(uname)" = GNU \
+    || skip_ 'not running on GNU/Hurd'
+}
+
 sanitize_path_