RTSP: improved RTP parser

- fix HTTP header parsing to report incomplete
  lines it buffers as consumed!
- re-implement the RTP parser for interleave RTP
  messages for robustness. It is now keeping its
  state at the connection
- RTSP protocol handler "readwrite" implementation
  now tracks if the response is before/in/after
  header parsing or "in" a bod by calling
  "Curl_http_readwrite_headers()" itself. This
  allows it to know when non-RTP bytes are "junk"
  or HEADER or BODY.
- tested with #12035 and various small receive
  sizes where current master fails

Closes #12052

lib/http.c

@@ -3986,6 +3986,7 @@ CURLcode Curl_http_readwrite_headers(struct Curl_easy *data,
   char *end_ptr;
 
   /* header line within buffer loop */
+  *stop_reading = FALSE;
   do {
     size_t rest_length;
     size_t full_length;
@@ -4023,7 +4024,7 @@ CURLcode Curl_http_readwrite_headers(struct Curl_easy *data,
           break;
         }
       }
-
+      *nread = 0;
       break; /* read more and try again */
     }