[det]tcp_fcntl(+Stream,
+Action, ?Argument)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.