mail/mailman3: create port

Mailman is free software for managing electronic mail discussion
and e-newsletter lists. Mailman is integrated with the web, making
it easy for users to manage their accounts and for list owners to
administer their lists. Mailman supports built-in archiving, automatic
bounce processing, content filtering, digest delivery, spam filters,
and more.

WWW: http://www.list.org/

PR: 225543
Submitted by: Charlie Li <ml+freebsd@vishwin.info> (with changes from pi)