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/pop3.c

@@ -1742,9 +1742,8 @@ static CURLcode pop3_write(struct Curl_easy *data, const char *str,
 
     /* Did we have a partial match which has subsequently failed? */
     if(prev && prev >= pop3c->eob) {
-      /* Strip can only be non-zero for the very first mismatch after CRLF
-         and then both prev and strip are equal and nothing will be output
-         below */
+      /* Strip can only be non-zero for the first mismatch after CRLF and
+         then both prev and strip are equal and nothing will be output below */
       while(prev && pop3c->strip) {
         prev--;
         pop3c->strip--;