ldap: support insecure mode for Windows native LDAP

- Bypass cert verification if verifypeer is disabled.

Prior to this change libcurl lacked the ability to bypass certificate
verification for Windows native LDAP (USE_WIN32_LDAP). A comment said
"Win32 LDAP SDK does not support insecure mode without CA!" but I found
that we can bypass the check by setting a verify callback to override
Windows' internal verify check.

Closes https://github.com/curl/curl/pull/22152

lib/ldap.c

@@ -251,6 +251,17 @@ static bool ldap_value_needs_base64(const char *attr, size_t attr_len,
   return FALSE;
 }
 
+#ifdef USE_WIN32_LDAP
+static BOOLEAN bypass_cert_verify(PLDAP Connection,
+                                  PCCERT_CONTEXT *ppServerCert)
+{
+  (void)Connection;
+  CertFreeCertificateContext(*ppServerCert);
+  /* approve any certificate since the verification is set to bypass */
+  return TRUE;
+}
+#endif
+
 static CURLcode ldap_do(struct Curl_easy *data, bool *done)
 {
   CURLcode result = CURLE_OK;
@@ -340,8 +351,18 @@ static CURLcode ldap_do(struct Curl_easy *data, bool *done)
   if(ldap_ssl) {
 #ifdef HAVE_LDAP_SSL
 #ifdef USE_WIN32_LDAP
-    /* Win32 LDAP SDK does not support insecure mode without CA! */
+    /* Win32 LDAP uses the Windows CA store to verify certificates */
     ldap_set_option(server, LDAP_OPT_SSL, LDAP_OPT_ON);
+    if(!conn->ssl_config.verifypeer) {
+      if(conn->ssl_config.verifyhost) {
+        failf(data, "LDAP local: host verification cannot be enabled when "
+              "peer verification is disabled for Windows native LDAP");
+        result = CURLE_NOT_BUILT_IN;
+        goto quit;
+      }
+      ldap_set_option(server, LDAP_OPT_SERVER_CERTIFICATE,
+                      (void *)(uintptr_t)bypass_cert_verify);
+    }
 #else /* !USE_WIN32_LDAP */
     int ldap_option;
     const char *ldap_ca = conn->ssl_config.CAfile;