Write out-of-band data to a stream.
Writes out-of-band data to a stream and returns how many bytes
that were actually written. It can be less than the size of the
given data if some data was written successfully and then
something went wrong.
-1 is returned if something went wrong and no bytes were written.
If only some data was written due to an error and that error
persists, then a later call to write_oob() fails and returns
-1.
If everything went fine, a call to errno() directly afterwards
returns zero.
If more than one argument is given, sprintf() is used to format
them.