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.

systemd/backend.go

@@ -0,0 +1,37 @@
+package systemd
+
+import "github.com/warkanum/unitdore/config"
+
+// Backend is the systemd implementation of the init interface.
+type Backend struct{}
+
+func (Backend) Name() string { return "systemd" }
+
+func (Backend) ServiceName(u config.Unit, prefix, suffix string) string {
+	return ServiceName(u, prefix, suffix)
+}
+
+func (Backend) UnitPath(u config.Unit, prefix, suffix string) (string, error) {
+	return UnitPath(u, prefix, suffix)
+}
+
+func (Backend) Generate(u config.Unit, prefix, suffix string) (string, error) {
+	return Generate(u, prefix, suffix)
+}
+
+func (Backend) Install(u config.Unit, prefix, suffix string) error { return Install(u, prefix, suffix) }
+func (Backend) Uninstall(u config.Unit, prefix, suffix string) error {
+	return Uninstall(u, prefix, suffix)
+}
+func (Backend) Enable(u config.Unit, prefix, suffix string) error  { return Enable(u, prefix, suffix) }
+func (Backend) Disable(u config.Unit, prefix, suffix string) error { return Disable(u, prefix, suffix) }
+func (Backend) Start(u config.Unit, prefix, suffix string) error   { return Start(u, prefix, suffix) }
+func (Backend) Stop(u config.Unit, prefix, suffix string) error    { return Stop(u, prefix, suffix) }
+func (Backend) Status(u config.Unit, prefix, suffix string) string { return Status(u, prefix, suffix) }
+
+func (Backend) IsInstalled(u config.Unit, prefix, suffix string) bool {
+	return IsInstalled(u, prefix, suffix)
+}
+
+// SupportsLinger reports that user units need loginctl linger to start at boot.
+func (Backend) SupportsLinger() bool { return true }