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/gtls.c

@@ -1309,7 +1309,7 @@ static CURLcode pkp_pin_peer_pubkey(struct Curl_easy *data,
   if(key)
     gnutls_pubkey_deinit(key);
 
-  Curl_safefree(buff1);
+  curlx_safefree(buff1);
 
   return result;
 }