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/ftpupload.c

@@ -90,7 +90,7 @@ int main(void)
   /* get a FILE * of the file */
   hd_src = fopen(LOCAL_FILE, "rb");
   if(!hd_src) {
-    printf("Couldn't open '%s': %s\n", LOCAL_FILE, strerror(errno));
+    printf("Could not open '%s': %s\n", LOCAL_FILE, strerror(errno));
     return 2;
   }