Add test-in-container infrastructure.

* Makefile (testroot.pristine): New rules to initialize the
test-in-container "testroot".
* Makerules (all-testsuite): Add tests-container.
* Rules (tests-expected): Add tests-container.
(binaries-all-tests): Likewise.
(tests-container): New, run these tests in the testroot container.
* support/Makefile (others): Add *-container, support_paths.c,
xmkdirp, and links-dso-program.
* support/links-dso-program-c.c: New.
* support/links-dso-program.cc: New.
* support/test-container.c: New.
* support/shell-container.c: New.
* support/echo-container.c: New.
* support/true-container.c: New.
* support/xmkdirp.c: New.
* support/xsymlink.c: New.
* support/support_paths.c: New.
* support/support.h: Add support paths prototypes.
* support/xunistd.h: Add xmkdirp () and xsymlink ().

* nss/tst-nss-test3.c: Convert to test-in-container.
* nss/tst-nss-test3.root/: New.

support/true-container.c

@@ -0,0 +1,26 @@
+/* Minimal /bin/true for in-container use.
+   Copyright (C) 2018 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library 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
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+/* Implements the in-container /bin/true, which always returns true
+   (0).  */
+
+int
+main (void)
+{
+  return 0;
+}