lib: replace conn [write]sockfd members by index

The `connectdata` members `sockfd` and `writesockfd` needed to by either
CURL_SOCKET_BAD or a copy of one of `conn->sock[2]`. When equal to one,
that index was used to send/recv/poll the proper socket or connection
filter chain.

Replace those with `send_idx` and `recv_idx` which are either -1, 0 or 1
to indicate which socket/filter to send/receive on.

Closes #18179

lib/http.c

@@ -2704,8 +2704,6 @@ CURLcode Curl_http(struct Curl_easy *data, bool *done)
       if(result)
         goto fail;
       info_version = "HTTP/2";
-      /* There is no ALPN here, but the connection is now definitely h2 */
-      conn->httpversion_seen = 20;
     }
     else
       info_version = "HTTP/1.x";