multi: add xfer_buf to multi handle

- can be borrowed by transfer during recv-write operation
- needs to be released before borrowing again
- adjustis size to `data->set.buffer_size`
- used in transfer.c readwrite_data()

Closes #12805

lib/setopt.c

@@ -2210,9 +2210,6 @@ CURLcode Curl_vsetopt(struct Curl_easy *data, CURLoption option, va_list param)
      * The application kindly asks for a differently sized receive buffer.
      * If it seems reasonable, we'll use it.
      */
-    if(data->state.buffer)
-      return CURLE_BAD_FUNCTION_ARGUMENT;
-
     arg = va_arg(param, long);
 
     if(arg > READBUFFER_MAX)