feat: support setup without domain, with only an IPv4 address (#963)

* dovecot: enable login names with square brackets

* config: make IPv4-only relays use self-signed TLS certs

* postfix: make delivery for IP-only relays work

* cmdeploy: skip DNS checks for IPv4 only relays

* www: generate dclogin codes for IPv4-only relays

* opendkim: disable DKIM signing on ipv4-only relays

* get delivery working

* get tests working on IPv4 only machine

* doc: document IPv4-only relays

* dns: warn if mail_domain is an IP, instead of checking DNS

* config: validate domains when formatting them

* ci: add cmlxc testing for no-DNS relays

* ci: run no-dns and normal CI in parallel

* retain "config.mail_domain" as the domain part of @ email addresses, so for ipv4 relays  "[1.2.3.4]" and introduce config.ipv4_relay and config.mail_domain_bare helpers.

* ci: migrate from --no-dns to --type ipv4 for cmlxc compatibility

* cleanup dead code, fix docs, fixate cmlxc version

---------

Co-authored-by: missytake <missytake@systemli.org>

doc/source/getting_started.rst

@@ -14,8 +14,6 @@ Minimal requirements and prerequisites
 
 You will need the following:
 
--  Control over a domain through a DNS provider of your choice.
-
 -  A Debian 12 **deployment server** with reachable SMTP/SUBMISSIONS/IMAPS/HTTPS ports.
    IPv6 is encouraged if available. Chatmail relay servers only require
    1GB RAM, one CPU, and perhaps 10GB storage for a few thousand active
@@ -28,6 +26,11 @@ You will need the following:
    (An ed25519 private key is required due to an `upstream bug in
    paramiko <https://github.com/paramiko/paramiko/issues/2191>`_)
 
+-  Control over a domain through a DNS provider of your choice
+   (there is experimental support for :ref:`IP-only relays <iponly>`).
+
+
+.. _setup:
 
 Setup with ``scripts/cmdeploy``
 -------------------------------------