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

lib/headers.c

@@ -51,7 +51,7 @@ static void copy_header_external(struct Curl_header_store *hs,
   h->index = index;
   /* this will randomly OR a reserved bit for the sole purpose of making it
      impossible for applications to do == comparisons, as that would otherwise
-     be very tempting and then lead to the reserved bits not being reserved
+     be tempting and then lead to the reserved bits not being reserved
      anymore. */
   h->origin = (unsigned int)(hs->type | (1 << 27));
   h->anchor = e;