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>

chatmaild/src/chatmaild/tests/plugin.py

@@ -31,6 +31,11 @@ def example_config(make_config):
     return make_config("chat.example.org")
 
 
+@pytest.fixture
+def ipv4_config(make_config):
+    return make_config("1.3.3.7")
+
+
 @pytest.fixture
 def maildomain(example_config):
     return example_config.mail_domain