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/http2-download.c

@@ -90,7 +90,7 @@ static void dump(const char *text, int num, unsigned char *ptr,
       }
       fprintf(stderr, "%c",
               (ptr[i + c] >= 0x20) && (ptr[i + c] < 0x80) ? ptr[i + c] : '.');
-      /* check again for 0D0A, to avoid an extra \n if it's at width */
+      /* check again for 0D0A, to avoid an extra \n if it is at width */
       if(nohex && (i + c + 2 < size) && ptr[i + c + 1] == 0x0D &&
          ptr[i + c + 2] == 0x0A) {
         i += (c + 3 - width);