security.c: Fix Curl_sec_login after rewrite.

Curl_sec_login was returning the opposite result that the code in ftp.c
was expecting. Simplified the return code (using a CURLcode) so to see
more clearly what is going on.

lib/ftp.c

@@ -2424,7 +2424,7 @@ static CURLcode ftp_statemach_act(struct connectdata *conn)
            set a valid level */
         Curl_sec_request_prot(conn, data->set.str[STRING_KRB_LEVEL]);
 
-        if(Curl_sec_login(conn) != 0)
+        if(Curl_sec_login(conn) != CURLE_OK)
           infof(data, "Logging in with password in cleartext!\n");
         else
           infof(data, "Authentication successful\n");