mod_net_proxy: Use safer util.format for generating description string (thanks tom) It appears that server_event may emit port numbers (src_port() in this case) as strings, rather than integers. This causes string.format() to throw an error. Prosody's util.format is a more forgiving formatter and won't throw an error. The server_event behaviour may be best fixed for the sake of consistency, though.

# Introduction

This module sends a [version query][xep0092] to clients when they
connect, and logs the response, allowing statistics of client usage to
be recorded.

Note that since this is per connection, there will be a bias towards
mobile clients on bad connections.

## Example

    info    Running Running Swift version 4.0