namespace-util: modernize fd_is_namespace() and is_our_namespace()

- Make fd_is_namespace() take NamespaceType
- Drop support for kernel without NS_GET_NSTYPE (< 4.11)
- Port is_our_namespace() to namespace_open_by_type()
  (preparation for later commits, where the latter
  would go by pidfd if available, avoiding procfs)

src/nspawn/nspawn.c

@@ -5249,12 +5249,10 @@ static int run_container(
                 if (child_netns_fd < 0)
                         return log_error_errno(errno, "Cannot open file %s: %m", arg_network_namespace_path);
 
-                r = fd_is_ns(child_netns_fd, CLONE_NEWNET);
-                if (r == -EUCLEAN)
-                        log_debug_errno(r, "Cannot determine if passed network namespace path '%s' really refers to a network namespace, assuming it does.", arg_network_namespace_path);
-                else if (r < 0)
+                r = fd_is_namespace(child_netns_fd, NAMESPACE_NET);
+                if (r < 0)
                         return log_error_errno(r, "Failed to check %s fs type: %m", arg_network_namespace_path);
-                else if (r == 0)
+                if (r == 0)
                         return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
                                                "Path %s doesn't refer to a network namespace, refusing.", arg_network_namespace_path);
         }