test: Add framework for containerized testing

On Linux we can potentially use user and UTS namespaces to run  a test
in a pseudo-container with:
 - arbitrary filesystem (e.g. /etc/resolv.conf, /etc/nsswitch.conf, /etc/hosts)
 - arbitrary hostname/domainname.

Include a first pass at the framework code to allow this, along with a
first test case that uses the container.

m4/ax_check_user_namespace.m4

@@ -0,0 +1,54 @@
+# -*- Autoconf -*-
+
+# SYNOPSIS
+#
+#   AX_CHECK_USER_NAMESPACE
+#
+# DESCRIPTION
+#
+#   This macro checks whether the local system supports Linux user namespaces.
+#   If so, it calls AC_DEFINE(HAVE_USER_NAMESPACE).
+
+AC_DEFUN([AX_CHECK_USER_NAMESPACE],[dnl
+ AC_CACHE_CHECK([whether user namespaces are supported],
+  ax_cv_user_namespace,[
+  AC_LANG_PUSH([C])
+  AC_RUN_IFELSE([AC_LANG_SOURCE([[
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <sched.h>
+#include <signal.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+
+int userfn(void *d) {
+  usleep(100000);  /* synchronize by sleep */
+  return (getuid() != 0);
+}
+char userst[1024*1024];
+int main() {
+  char buffer[1024];
+  int rc, status, fd;
+  pid_t child = clone(userfn, userst + 1024*1024, CLONE_NEWUSER|SIGCHLD, 0);
+  if (child < 0) return 1;
+
+  sprintf(buffer, "/proc/%d/uid_map", child);
+  fd = open(buffer, O_CREAT|O_WRONLY|O_TRUNC, 0755);
+  sprintf(buffer, "0 %d 1\n", getuid());
+  write(fd, buffer, strlen(buffer));
+  close(fd);
+
+  rc = waitpid(child, &status, 0);
+  if (rc <= 0) return 1;
+  if (!WIFEXITED(status)) return 1;
+  return WEXITSTATUS(status);
+}
+  ]])],[ax_cv_user_namespace=yes], [ax_cv_user_namespace=no])
+ AC_LANG_POP([C])
+ ])
+ if test "$ax_cv_user_namespace" = yes; then
+   AC_DEFINE([HAVE_USER_NAMESPACE],[1],[Whether user namespaces are available])
+ fi
+]) # AX_CHECK_USER_NAMESPACE

m4/ax_check_uts_namespace.m4

@@ -0,0 +1,76 @@
+# -*- Autoconf -*-
+
+# SYNOPSIS
+#
+#   AX_CHECK_UTS_NAMESPACE
+#
+# DESCRIPTION
+#
+#   This macro checks whether the local system supports Linux UTS namespaces.
+#   Also requires user namespaces to be available, so that non-root users
+#   can enter the namespace.
+#   If so, it calls AC_DEFINE(HAVE_UTS_NAMESPACE).
+
+AC_DEFUN([AX_CHECK_UTS_NAMESPACE],[dnl
+ AC_CACHE_CHECK([whether UTS namespaces are supported],
+  ax_cv_uts_namespace,[
+  AC_LANG_PUSH([C])
+  AC_RUN_IFELSE([AC_LANG_SOURCE([[
+#define _GNU_SOURCE
+#include <sched.h>
+#include <signal.h>
+#include <stdio.h>
+#include <string.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+
+int utsfn(void *d) {
+  char buffer[1024];
+  const char *name = "autoconftest";
+  int rc = sethostname(name, strlen(name));
+  if (rc != 0) return 1;
+  gethostname(buffer, 1024);
+  return (strcmp(buffer, name) != 0);
+}
+
+char st2[1024*1024];
+int fn(void *d) {
+  pid_t child;
+  int rc, status;
+  usleep(100000);  /* synchronize by sleep */
+  if (getuid() != 0) return 1;
+  child = clone(utsfn, st2 + 1024*1024, CLONE_NEWUTS|SIGCHLD, 0);
+  if (child < 0) return 1;
+  rc = waitpid(child, &status, 0);
+  if (rc <= 0) return 1;
+  if (!WIFEXITED(status)) return 1;
+  return WEXITSTATUS(status);
+}
+char st[1024*1024];
+int main() {
+  char buffer[1024];
+  int rc, status, fd;
+  pid_t child = clone(fn, st + 1024*1024, CLONE_NEWUSER|SIGCHLD, 0);
+  if (child < 0) return 1;
+
+  sprintf(buffer, "/proc/%d/uid_map", child);
+  fd = open(buffer, O_CREAT|O_WRONLY|O_TRUNC, 0755);
+  sprintf(buffer, "0 %d 1\n", getuid());
+  write(fd, buffer, strlen(buffer));
+  close(fd);
+
+  rc = waitpid(child, &status, 0);
+  if (rc <= 0) return 1;
+  if (!WIFEXITED(status)) return 1;
+  return WEXITSTATUS(status);
+}
+]])
+  ],[ax_cv_uts_namespace=yes], [ax_cv_uts_namespace=no])
+ AC_LANG_POP([C])
+ ])
+ if test "$ax_cv_uts_namespace" = yes; then
+   AC_DEFINE([HAVE_UTS_NAMESPACE],[1],[Whether UTS namespaces are available])
+ fi
+]) # AX_CHECK_UTS_NAMESPACE