Function SilcSFTPExtendedCallback
SYNOPSIS
typedef void (*SilcSFTPExtendedCallback)(SilcSFTP sftp,
SilcSFTPStatus status,
const unsigned char *data,
uint32 data_len,
void *context);
DESCRIPTION
Extended request callback is called when client sends extended
request to the server. The `data' is arbitrary data returned by the
server and its encoding is the extended request specific.