urldata: merge "struct DynamicStatic" into "struct UrlState"

Both were used for the same purposes and there was no logical separation
between them. Combined, this also saves 16 bytes in less holes in my
test build.

Closes #6798

lib/multi.c

@@ -2164,7 +2164,7 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
           data->state.errorbuf = FALSE;
           if(!newurl)
             /* typically for HTTP_1_1_REQUIRED error on first flight */
-            newurl = strdup(data->change.url);
+            newurl = strdup(data->state.url);
           /* if we are to retry, set the result to OK and consider the request
              as done */
           retry = TRUE;