ldap: tidy-up types, fix error code confusion

ldap: tidy-up types, fix error code confusion

- fix `CURLcode` vs. LDAP result code confusion.
  Return `LDAP_NO_MEMORY` when `Curl_create_sspi_identity()` fails,
  since it can only return `CURLE_OUT_OF_MEMORY` as error.
- use `ULONG` for result code on Windows. Drop casts.
- use portable `curl_ldap_num_t`. Drop casts.
- replace magic number 0 with `LDAP_SUCCESS`.
- compare with `LDAP_SUCCESS` instead of assuming non-zero.
  (where necessary.)
- add/fix `#endif` comments.
- fix indentation.

Closes #18888