Enforce a message size limit

[luk3yx]

Forces all messages to be at most 512 bytes, to stop cmd help all from the irc_commands DoSing the server (making it quit with the message RecvQ exceeded).

This is done in the core IRC mod so it limits everything rather than just irc_commands.

Credit to @bigfoot547 for finding this vulnerability.

[sofar]

Yes, but, can we please maybe get some logging in the console as well if players want to cause floods this way? Silently dropping output may also cause unintentional side effects, so we should consider replying back with an error message and throttling the user for at least a few seconds, too.

[luk3yx]

What about adding ... to the end of long messages?

PRIVMSG #channel :A really long testing messag...

[sofar]

I'd almost favor dropping the message and sending an error to the client. That may help prevent floods.

[luk3yx]

That would need a change somewhere else, there may be rouge irc.say()-s in other mods that need fixing too.