feat(linux): Notify systemd when we've started (#3628)

Regardless of `FIREZONE_DNS_CONTROL`, always try to notify systemd that
we've started.

I had accidentally conflated the idea of running as a systemd service
with the idea of using systemd to control DNS. They're separate, but
I'll keep the service unit in here and always use `sd-notify` since it
should be harmless to use even in Alpine.

~~If `FIREZONE_DNS_CONTROL` is `systemd-resolved`, try to notify systemd
that we've finished startup and the tunnel is ready.~~

Also adds a CI test, including a systemd service file that is **not**
ready for general use.
Ready for review once it's green.

---------

Signed-off-by: Reactor Scram <ReactorScram@users.noreply.github.com>
Co-authored-by: Thomas Eizinger <thomas@eizinger.io>

.github/workflows/ci.yml

@@ -192,6 +192,7 @@ jobs:
           direct-ping-portal-relay-down,
           dns-etc-resolvconf,
           dns-nm,
+          systemd/dns-systemd-resolved,
         ]
     steps:
       - uses: actions/checkout@v4