Packaging: the binary and package are inbuxa, INBUXA_* settings with STALWART_* fallback

- crates/main: package and [[bin]] renamed to inbuxa; homepage inbuxa.org;
  license AGPL-3.0-only (upstream is dual; the fork takes the AGPL).
- types::branding::env_var reads INBUXA_<name>, falling back to
  STALWART_<name> with a warning, for all nine server settings.
  STALWART_APP_ and STALWART_SPAM_* storage keys are unchanged.
- New-install default paths /var/lib/inbuxa and /var/log/inbuxa.
- Dockerfiles, systemd unit, launchd plist and AppArmor profile renamed.
- Upstream's .github moved to .github-upstream so none of it runs.
- install.sh stubbed: upstream's would install Stalwart.
- Two missed brand strings: the SMTP Received header and the utils user agent.

resources/systemd/inbuxa.service

@@ -0,0 +1,21 @@
+[Unit]
+Description=INBUXA Server
+Conflicts=postfix.service sendmail.service exim4.service
+ConditionPathExists=__PATH__/etc/config.json
+After=network-online.target
+
+[Service]
+Type=simple
+LimitNOFILE=65536
+KillMode=process
+KillSignal=SIGINT
+Restart=on-failure
+RestartSec=5
+ExecStart=__PATH__/bin/inbuxa --config=__PATH__/etc/config.json
+SyslogIdentifier=inbuxa
+User=inbuxa
+Group=inbuxa
+AmbientCapabilities=CAP_NET_BIND_SERVICE
+
+[Install]
+WantedBy=multi-user.target