Revert "xwayland: Don't dup() displayfd pipe"

This reverts commit 4aa885d4af.

Turns out the problem was not about dupping fds at all, but calling
non-async-signal-safe functions like strdup() between fork() and exec()
in the child process.

For more discussion, see:
https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/941#note_1457053

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

compositor/xwayland.c

@@ -135,6 +135,11 @@ spawn_xserver(void *user_data, const char *display, int abstract_fd, int unix_fd
 		return 1;
 	}
 
+	if (os_fd_set_cloexec(display_fd[1]) != 0) {
+		weston_log("failed setting Xwayland end of displayfd as cloexec\n");
+		return 1;
+	}
+
 	pid = fork();
 	switch (pid) {
 	case 0:
@@ -149,16 +154,12 @@ spawn_xserver(void *user_data, const char *display, int abstract_fd, int unix_fd
 		ret &= dup_fd_to_string(abstract_fd_str, abstract_fd);
 		ret &= dup_fd_to_string(unix_fd_str, unix_fd);
 		ret &= dup_fd_to_string(wm_fd_str, wm[1]);
+		ret &= dup_fd_to_string(display_fd_str, display_fd[1]);
 		if (!ret)
 			_exit(EXIT_FAILURE);
 
 		setenv("WAYLAND_SOCKET", wayland_socket_str, 1);
 
-		if (snprintf(display_fd_str, sizeof(display_fd_str), "%d",
-			     display_fd[1]) <= 0) {
-			_exit(EXIT_FAILURE);
-		}
-
 		if (execl(xserver,
 			  xserver,
 			  display,