CURLOPT_BUFFERSIZE: support enlarging receive buffer

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