Add SMTPUTF8/Email Address Internationalization (EAI) support

[wornet-aer]

It would be pretty neat, if qpsmtpd could support SMTPUTF8. Postfix already has it and it seems like some other MTAs out there also start to support UTF-8 in email headers and addresses.

Maybe these resources can help implementing it:

• http://tools.ietf.org/html/rfc6531
• http://tools.ietf.org/html/rfc6532
• http://tools.ietf.org/html/rfc6533
• http://www.postfix.org/SMTPUTF8_README.html
• https://rant.gulbrandsen.priv.no/eai-implementation-notes

Unfortunately I'm not yet familiar enough with the qpsmtpd core to implement this myself.

[abh]

I suspect you'd get pretty far by adding tests to t/addresses.t and then working in Qpsmtpd::Address. Everything else should be using that abstraction.

[msimerson]

You might find haraka/Haraka#3289 useful as a reference.