libct: use chmod instead of umask

Umask is problematic for Go programs as it affects other goroutines
(see [1] for more details).

Instead of using it, let's just prop up with Chmod.

Note this patch misses the MkdirAll call in createDeviceNode. Since the
runtime spec does not say anything about creating intermediary
directories for device nodes, let's assume that doing it via mkdir with
the current umask set is sufficient (if not, we have to reimplement
MkdirAll from scratch, with added call to os.Chmod).

[1] https://github.com/opencontainers/runc/pull/3563#discussion_r990293788

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/console_linux.go

@@ -9,13 +9,15 @@ import (
 // mount initializes the console inside the rootfs mounting with the specified mount label
 // and applying the correct ownership of the console.
 func mountConsole(slavePath string) error {
-	oldMask := unix.Umask(0o000)
-	defer unix.Umask(oldMask)
 	f, err := os.Create("/dev/console")
 	if err != nil && !os.IsExist(err) {
 		return err
 	}
 	if f != nil {
+		// Ensure permission bits (can be different because of umask).
+		if err := f.Chmod(0o666); err != nil {
+			return err
+		}
 		f.Close()
 	}
 	return mount(slavePath, "/dev/console", "bind", unix.MS_BIND, "")