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/curlx/fopen.c

@@ -142,7 +142,7 @@ static bool fix_excessive_path(const TCHAR *in, TCHAR **out)
   else if(!wcsncmp(fbuf, L"\\\\.\\", 4))
     fbuf[2] = '?';
   else if(!wcsncmp(fbuf, L"\\\\.", 3) || !wcsncmp(fbuf, L"\\\\?", 3)) {
-    /* Unexpected, not UNC. The formatting doc doesn't allow this AFAICT. */
+    /* Unexpected, not UNC. The formatting doc does not allow this AFAICT. */
     goto cleanup;
   }
   else {

lib/curlx/inet_ntop.c

@@ -160,7 +160,7 @@ static char *inet_ntop6(const unsigned char *src, char *dst, size_t size)
       break;
     }
     else {
-      /* Lower-case digits. Can't use the set from mprintf.c since this
+      /* Lower-case digits. Cannot use the set from mprintf.c since this
          needs to work as a curlx function */
       static const unsigned char ldigits[] = "0123456789abcdef";