Read out-of-band data from a stream.
Attempts to read len bytes of out-of-band data from the stream,
and returns it as a string. If something goes wrong, zero is returned.
If a one is given as the second argument to read_oob() , only
as many bytes of out-of-band data as are currently available will be
returned.
If no arguments are given, read_oob() will read to the end of
the stream.