Write data to a channel stream. All channel streams have one standard I/O
substream (stream_id == 0), and may have up to 2^32 extended data streams as
identified by the selected stream_id. The SSH2 protocol currently
defines a stream ID of 1 to be the stderr substream.
channel - active channel stream to write to.
stream_id - substream ID number (e.g. 0 or SSH_EXTENDED_DATA_STDERR)
Actual number of bytes written or negative on failure.
LIBSSH2_ERROR_EAGAIN when it would otherwise block. While
LIBSSH2_ERROR_EAGAIN is a negative number, it isn't really a failure per se.
ERRORS
LIBSSH2_ERROR_ALLOC - An internal memory allocation call failed.
LIBSSH2_ERROR_SOCKET_SEND - Unable to send data on socket.
LIBSSH2_ERROR_CHANNEL_CLOSED - The channel has been closed.
LIBSSH2_ERROR_CHANNEL_EOF_SENT - The channel has been requested to be
closed.