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/hostip.c

@@ -956,7 +956,7 @@ out:
     /* we got a response, create a dns entry, add to cache, return */
     dns = Curl_dnscache_mk_entry(data, addr, hostname, 0, port, FALSE);
     if(!dns || Curl_dnscache_add(data, dns)) {
-      /* this is OOM or similar, don't store such negative resolves */
+      /* this is OOM or similar, do not store such negative resolves */
       keep_negative = FALSE;
       goto error;
     }
@@ -1393,7 +1393,7 @@ CURLcode Curl_loadhostpairs(struct Curl_easy *data)
       error = FALSE;
 err:
       if(error) {
-        failf(data, "Couldn't parse CURLOPT_RESOLVE entry '%s'",
+        failf(data, "Could not parse CURLOPT_RESOLVE entry '%s'",
               hostp->data);
         Curl_freeaddrinfo(head);
         return CURLE_SETOPT_OPTION_SYNTAX;