feat: OpenRC and runit init backends behind an Init interface

Extract init-neutral unit resolution into service/, add initsys.Init with systemd, openrc and runit backends, selected by 'init:' config or autodetect. OpenRC and runit are root only; rootless units run as system services as that user. Add docs and generated examples for all three.

cmd/syncup.go

@@ -9,7 +9,7 @@ import (
 	"github.com/spf13/cobra"
 	"github.com/warkanum/unitdore/config"
 	"github.com/warkanum/unitdore/runtime"
-	"github.com/warkanum/unitdore/systemd"
+	"github.com/warkanum/unitdore/service"
 	"github.com/warkanum/unitdore/sysuser"
 )
 
@@ -221,7 +221,7 @@ func mergeDiscovered(cfg *config.Config, containers []runtime.Container, compose
 // unitExists checks whether a unit's container or stack still exists.
 func unitExists(u config.Unit) (bool, error) {
 	if u.IsStack() {
-		if systemd.ComposeAvailable(u) {
+		if service.ComposeAvailable(u) {
 			return true, nil
 		}
 		return (&runtime.Podman{}).PodExists(u.User, runtime.PodName(u.Project))