Add Clone for ShmemConf

[aj3n]

    let shm_conf = ShmemConf::new()
        .size(4096)
        .os_id("foo")
        .flink("bar");
    let mut shm = shm_conf
        .clone()
        .create()
        .or_else(|_| shm_conf.open())
        .unwrap();

It may be useful in occasions like this.