badwords: check indented lines in source code, fix fallouts

- badwords.pl: add `-a` option to check all lines in source code files.
  Before this patch indented lines were skipped (to avoid Markdown code
  fences.)
- GHA/checksrc: use `-a` when verifying the source code.
- GHA/checksrc: disable `So` and `But` rules for source code.
- GHA/checksrc: add docs/examples to the verified sources.
- badwords.txt: delete 4 duplicates.
- badwords.txt: group and sort contractions.
- badwords.txt: allow ` url = `, `DIR`, `<file name`.

Closes #19536

docs/examples/urlapi.c

@@ -42,7 +42,7 @@ int main(void)
   if(res)
     return (int)res;
 
-  /* init Curl URL */
+  /* init curl URL */
   urlp = curl_url();
   uc = curl_url_set(urlp, CURLUPART_URL,
                     "http://example.com/path/index.html", 0);