Close the connection. Both the read and write ends are always
closed - the argument how is only for Stdio.File
compatibility and must be either "rw"
or 0
.
If clean_close is set then close messages are exchanged to shut
down the SSL connection but not the underlying stream. It may then
continue to be used for other communication afterwards. The
default is to send a close message and then close the stream
without waiting for a response.
Returns zero and sets the errno to System.EBADF if the
connection already is closed. Other I/O errors are thrown.