mail/py-imaplib2: Threaded Python IMAP4 client (new port)

Based on RFC 3501 and original imaplib module.

This is a version of imaplib that uses threads to allow full use of
the IMAP4 concurrency features, and to de-couple a user of imaplib
from i/o lags, except where explicitly allowed.

PR: 262232
Author: Derek Schrock <dereks@lifeofadishwasher.com>
Differential Revision: https://reviews.freebsd.org/D34390