- Run `configtest` on reload, so it will fail if there are errors.

- Run `configtest` on start also, just for consistency.
  (No functional change, it already failed on bad config.)

- Designate the service as exec rather than simple.
  Slightly more correct, but in practice, pretty much identical.

Index: headscale-prebuilt-0.29.2/headscale.service
===================================================================
--- headscale-prebuilt-0.29.2.orig/headscale.service
+++ headscale-prebuilt-0.29.2/headscale.service
@@ -4,10 +4,12 @@ Description=headscale coordination serve
 X-Restart-Triggers=/etc/headscale/config.yaml
 
 [Service]
-Type=simple
+Type=exec
 User=headscale
 Group=headscale
+ExecStartPre=/usr/bin/headscale configtest
 ExecStart=/usr/bin/headscale serve
+ExecReload=/usr/bin/headscale configtest
 ExecReload=/usr/bin/kill -HUP $MAINPID
 Restart=always
 RestartSec=5
