reimplement recvmsg/sendmmsg

New implementation performs no allocations after all the necessary
structures are created, removes potentially unsound code that
was used by the old version (see below) and adds a bit more
documentation about bugs in how timeout is actually handled

```
    let timeout = if let Some(mut t) = timeout {
        t.as_mut() as *mut libc::timespec
    } else {
        ptr::null_mut()
    };
```