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/asyn-base.c

@@ -238,7 +238,7 @@ void Curl_async_destroy(struct Curl_easy *data,
 #ifndef CURL_DISABLE_DOH
     Curl_doh_cleanup(data, async);
 #endif
-    Curl_safefree(async);
+    curlx_safefree(async);
   }
 }