Some code (e.g. Curl_fillreadbuffer) assumes that this buffer is not
exceedingly tiny and will break if it is. This same check is already
done at run time in the CURLOPT_BUFFERSIZE option.
Replace use of fixed macro BUFSIZE to define the size of the receive
buffer. Reappropriate CURLOPT_BUFFERSIZE to include enlarging receive
buffer size. Upon setting, resize buffer if larger than the current
default size up to a MAX_BUFSIZE (512KB). This can benefit protocols
like SFTP.
Closes#1222
