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/support_paths.c

@@ -0,0 +1,51 @@
+/* Various paths that might be needed.
+   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/>.  */
+
+#include <support/support.h>
+#include <support/check.h>
+
+/* The idea here is to make various makefile-level paths available to
+   support programs, as canonicalized absolute paths.  */
+
+/* These point to the TOP of the source/build tree, not your (or
+   support's) subdirectory.  */
+#ifdef SRCDIR_PATH
+const char support_srcdir_root[] = SRCDIR_PATH;
+#else
+# error please -DSRCDIR_PATH=something in the Makefile
+#endif
+
+#ifdef OBJDIR_PATH
+const char support_objdir_root[] = OBJDIR_PATH;
+#else
+# error please -DOBJDIR_PATH=something in the Makefile
+#endif
+
+#ifdef INSTDIR_PATH
+/* Corresponds to the --prefix= passed to configure.  */
+const char support_install_prefix[] = INSTDIR_PATH;
+#else
+# error please -DINSTDIR_PATH=something in the Makefile
+#endif
+
+#ifdef LIBDIR_PATH
+/* Corresponds to the install's lib/ or lib64/ directory.  */
+const char support_libdir_prefix[] = LIBDIR_PATH;
+#else
+# error please -DLIBDIR_PATH=something in the Makefile
+#endif