Better type safety for mqueue

On some platforms, mqd_t is a pointer.  That means code like the below
can trigger a segfault.  Fix it by defining a Newtype around mqd_t that
prevents use-after-free and dangling pointer scenarios.

```rust
fn invalid_mqd_t() {
    let mqd: libc::mqd_t = std::ptr::null_mut();
    mq_close(mqd).unwrap();
}
```

Also, get test coverage for mqueue in CI on FreeBSD.

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