create main opensmtpd config

We are switching to OpenSMTPD because: "OpenSMTPD is easier to configure, has a smaller codebase and is part of the openBSD project which is quite security aware" - @freswa

smtpd.conf:

use table file for authentication
port 25 shall not be used for authenticated user access but by foreign MTAs only
port 465 with implicit tls to be used by the users
Domains: archlinux.org, master-keys.archlinux.org, aur.archlinux.org
check rdns by a opensmtpd filter
check fcrdns by a opensmtpd filter
check rdns for *dyn* and *dsl* pattern by a opensmtpd filter
rspamd integration with opensmtpd-filter-rspamd
lmtp delivery to dovecot

Edited Dec 29, 2020 by Kristian Klausen

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