Support time namespace

"time" namespace was introduced in Linux v5.6
support new time namespace to set boottime and monotonic time offset

Example runtime spec

"timeOffsets": {
    "monotonic": {
        "secs": 172800,
        "nanosecs": 0
    },
    "boottime": {
        "secs": 604800,
        "nanosecs": 0
    }
}

Signed-off-by: Chethan Suresh <chethan.suresh@sony.com>

libcontainer/configs/validate/validator.go

@@ -106,6 +106,12 @@ func namespaces(config *configs.Config) error {
 		}
 	}
 
+	if config.Namespaces.Contains(configs.NEWTIME) {
+		if _, err := os.Stat("/proc/self/timens_offsets"); os.IsNotExist(err) {
+			return errors.New("time namespaces aren't enabled in the kernel")
+		}
+	}
+
 	return nil
 }