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/rtsp.h

@@ -39,6 +39,12 @@ CURLcode Curl_rtsp_parseheader(struct Curl_easy *data, char *header);
 
 #endif /* CURL_DISABLE_RTSP */
 
+typedef enum {
+  RTP_PARSE_SKIP,
+  RTP_PARSE_CHANNEL,
+  RTP_PARSE_LEN,
+  RTP_PARSE_DATA
+} rtp_parse_st;
 /*
  * RTSP Connection data
  *
@@ -47,6 +53,9 @@ CURLcode Curl_rtsp_parseheader(struct Curl_easy *data, char *header);
 struct rtsp_conn {
   struct dynbuf buf;
   int rtp_channel;
+  size_t rtp_len;
+  rtp_parse_st state;
+  BIT(in_header);
 };
 
 /****************************************************************************