Enable UTF8=ACCEPT if the server supports ENABLE (and UTF=ACCEPT).

[arnt]

This is written so that net-imap has the option of enabling IMAP4REV2 instead, if the IMAP server has that. (IMAP4Rev2 includes the same syntax and functionality.)

With this PR, mikel-mail is able to read a message with UTF8 addresses from a compliant IMAP server, generate a reply and send it. Both net-imap and net-smtp contain the necessary code. At one line of code, this is the smallest such PR I've ever made ;)

(You may wonder about the call to .capable?. It's enough, see RFF5161 page 2. ENABLE is written to allow clients to send a fixed string; the server ignores what it can't enable.)