[NEW PORT]: mail/managesieve3: Implements an RFC-5804 Manage Sieve client

A pure Python client implementation of "A Protocol for Remotely
Managing Sieve Scripts", as defined in RFC-5804.

WWW: https://bitbucket.org/ericvsmith/managesieve3

Reviewed by: koobs, mat
Approved by: koobs (mentor)
Differential Revision: https://reviews.freebsd.org/D16699