lib: xfer_setup and non-blocking shutdown

- clarify Curl_xfer_setup() with RECV/SEND flags and different calls for
  which socket they operate on. Add a shutdown flag for secondary
  sockets
- change Curl_xfer_setup() calls to new functions
- implement non-blocking connection shutdown at the end of receiving or
  sending a transfer

Closes #13913

lib/http.c

@@ -2247,7 +2247,7 @@ CURLcode Curl_http_req_complete(struct Curl_easy *data,
 out:
   if(!result) {
     /* setup variables for the upcoming transfer */
-    Curl_xfer_setup(data, FIRSTSOCKET, -1, TRUE, FIRSTSOCKET);
+    Curl_xfer_setup1(data, CURL_XFER_SENDRECV, -1, TRUE);
   }
   return result;
 }