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/update.go

@@ -6,7 +6,7 @@ import (
 
 	"github.com/spf13/cobra"
 	"github.com/warkanum/unitdore/config"
-	"github.com/warkanum/unitdore/systemd"
+	"github.com/warkanum/unitdore/initsys"
 )
 
 var (
@@ -64,12 +64,16 @@ func runUpdate(cmd *cobra.Command, args []string) error {
 	}
 
 	prefix, suffix := cfg.Prefix, cfg.Suffix
+	ini, err := initsys.New(cfg)
+	if err != nil {
+		return err
+	}
 
-	if systemd.IsInstalled(*u, prefix, suffix) {
-		if err := systemd.Install(*u, prefix, suffix); err != nil {
+	if ini.IsInstalled(*u, prefix, suffix) {
+		if err := ini.Install(*u, prefix, suffix); err != nil {
 			return fmt.Errorf("recreating service file: %w", err)
 		}
-		path, _ := systemd.UnitPath(*u, prefix, suffix)
+		path, _ := ini.UnitPath(*u, prefix, suffix)
 		fmt.Printf("  ✓ service file recreated: %s\n", path)
 	}