MOTD centerprint fixes / changes:

MOTD centerprint fixes / changes:

* Kill initial MOTD centerprint (that appears on server connection) when we know it's showing (in PrintWelcomeMessage()), rather than doing it blindly every time the player joins the game: it fixes MOTD never expiring in Arena.
* Info button now instantly hides initial MOTD.
* If player pressed the info button to see the MOTD and immediately joined the game then don't care, just let the MOTD fade away normally (in 2 seconds); killing the MOTD as before caused the MOTD to pop up again after 2 seconds fading away