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/log_failed_transfers.c

@@ -140,8 +140,8 @@ static int mem_addf(struct mem *mem, const char *format, ...)
   /* we need about 100 chars or less to write 95% of lines */
   x = 128;
 
-  /* first try: there's probably enough memory to write everything.
-     second try: there's definitely enough memory to write everything. */
+  /* first try: there is probably enough memory to write everything.
+     second try: there is definitely enough memory to write everything. */
   for(i = 0; i < 2; ++i) {
     if(x < 0 || mem_need(mem, (size_t)x + 1) < 0)
       break;