socket-util: Introduce send_one_fd_iov() and receive_one_fd_iov()

socket-util: Introduce send_one_fd_iov() and receive_one_fd_iov()

These take a struct iovec to send data together with the passed FD.

The receive function returns the FD through an output argument. In case data is
received, but no FD is passed, the receive function will set the output
argument to -1 explicitly.

Update code in dynamic-user to use the new helpers.