Add support for EAI addresses

[jrlevine]

Email Address Internationalization (EAI) allows UTF-8 U-labels in domain names as well as the ASCII A-labels in the DNS. This turns U-labels in HELO, MAIL FROM, and RCPT TO addresses into A-labels before lookup. No change to the API or UI, but it'll now work with EAI mail. Tested on MacOS and FreeBSD. Needs the GNU libidn2 to do the U-label to A-label conversion.