http: separate response parsing from response action

- move code that triggers on end-of-response into separate function from
  parsing
- simplify some headp/headerlen usage
- add `httpversion` to SingleRequest to indicate the version of the
  current response

Closes #13134

lib/request.h

@@ -78,6 +78,7 @@ struct SingleRequest {
                                    first one */
   curl_off_t offset;            /* possible resume offset read from the
                                    Content-Range: header */
+  int httpversion;              /* Version in response (09, 10, 11, etc.) */
   int httpcode;                 /* error code from the 'HTTP/1.? XXX' or
                                    'RTSP/1.? XXX' line */
   int keepon;