This is patch 2 of 4 implementing support for SMTPUTF8 (RFC 6531).

Perform charset conversion from local to UTF-8 for both the user and
domain parts of the address.

If IDN is enabled and the options (added in the next patch) are turned
on, encode/decode the domain part.

Use the intl_checked and is_intl status bits to record the intl/local
status of the ADDRESS mailbox part.