We do this by simply creating a fd-managing type, e.g. UnixDatagram,
from the fd once it's created.

Also first tries to parse the path as that can fail without doing a
system call.

Now all the fd creating code follows the following pattern.

let raw_fd = create_fd();
let managed_fd = wrap(raw_fd);
// Additional system calls
return managed_fd

Do it in new_socket so that the higher levels don't have to worry about
it.

Don't agree with the `&path` -> `path`, but it's fine.