checksrc: disallow spaces before labels

Out of 415 labels throughout the code base, 86 of those labels were
not at the start of the line. Which means labels always at the start of
the line is the favoured style overall with 329 instances.

Out of the 86 labels not at the start of the line:
* 75 were indented with the same indentation level of the following line
* 8 were indented with exactly one space
* 2 were indented with one fewer indentation level then the following
  line
* 1 was indented with the indentation level of the following line minus
  three space (probably unintentional)

Co-Authored-By: Viktor Szakats

Closes #11134

docs/examples/parseurl.c

@@ -74,7 +74,7 @@ int main(void)
     curl_free(path);
   }
 
-  fail:
+fail:
   curl_url_cleanup(h); /* free url handle */
   return 0;
 }