curl_schannel.h: Use BUFSIZE as the initial buffer size if available

curl_schannel.h: Use BUFSIZE as the initial buffer size if available

Make the Schannel implementation use libcurl's default buffer size
for the initial received encrypted and decrypted data cache buffers.
The implementation still needs to handle more data since more data
might have already been received or decrypted during the handshake
or a read operation which needs to be cached for the next read.