Add a warning when using a proxy with UDP enabled

As per the Tox Client Standard.
<https://github.com/Tox/Tox-Client-Standard>

src/twc-profile.c

@@ -311,6 +311,14 @@ twc_profile_load(struct t_twc_profile *profile)
                        TOX_PROXY_TYPE_SOCKS5 ? "SOCKS5" :
                        NULL,
                        options.proxy_host, options.proxy_port);
+
+        if (options.udp_enabled)
+            weechat_printf(profile->buffer,
+                           "%swarning: Tox is configured to use a proxy, but "
+                           " UDP is not disabled. Your IP address may not be "
+                           "hidden.",
+                           weechat_prefix("error"),
+                           options.proxy_host, options.proxy_port);
     }
 
     // try loading data file