Over-long TLDs not recognised

[Arantor]

Applies to 2.0, seems to be present in 2.1, ref http://www.simplemachines.org/community/index.php?topic=527719.msg3742953#msg3742953

All I did was look for regexes containing {2,6} knowing that was the way TLD was checked. (I didn't bother fixing the MSN one in that topic, as MSN is now dead)

Provisionally, {2,16} should fix it, I don't know any TLD longer than 16 characters (but I do know some in the 13-14 character range)

This might be useful to patch in 2.0.10, but for 2.1 I'd strongly suggest at the very least abstracting it to a function called isValidEmail() or similar so that if the rules change again in future, it can be updated without problem. In a really ideal world, this function would also use filter_var to handle the email since PHP can probably do a better job of it than a single regex can.