progress: count amount of data "delivered" to application

... and apply the CURLOPT_MAXFILESIZE limit (if set) on that as well.
This effectively protects the user against "zip bombs".

Test case 1618 verifies using a 14 byte brotli payload that otherwise
explodes to 102400 zero bytes.

Closes #20787

src/tool_writeout.h

@@ -90,6 +90,7 @@ typedef enum {
   VAR_REFERER,
   VAR_REQUEST_SIZE,
   VAR_SCHEME,
+  VAR_SIZE_DELIVERED,
   VAR_SIZE_DOWNLOAD,
   VAR_SIZE_UPLOAD,
   VAR_SPEED_DOWNLOAD,