fcntl() call. Currently only suitable to deal
switch stream to non-blocking mode using:
tcp_fcntl(Stream, setfl, nonblock),
An attempt to read from a non-blocking stream while there is no
data available returns -1 (or end_of_file for read/1), but
at_end_of_stream/1 fails. On actual end-of-input,
at_end_of_stream/1 succeeds.