tidy-up: rename Curl_safefree()/tool_safefree() to curlx_safefree()

To sync names for the same macro logic between lib and src, and to move
it to the curlx namespace, to match `curlx_free()` that it's calling.

Closes #21151

lib/vtls/keylog.c

@@ -52,7 +52,7 @@ void Curl_tls_keylog_open(void)
           keylog_file_fp = NULL;
         }
       }
-      Curl_safefree(keylog_file_name);
+      curlx_safefree(keylog_file_name);
     }
   }
 }