Add support for Initialisation Scripts with rcS (#340)

* validate systemctl command

* validate systemctl command

* add rcS support

* only attempt to set environment if list is not empty

* check if service bin exists

service_systemd_linux.go

@@ -9,6 +9,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"os/exec"
 	"os/signal"
 	"path/filepath"
 	"regexp"
@@ -22,6 +23,9 @@ func isSystemd() bool {
 	if _, err := os.Stat("/run/systemd/system"); err == nil {
 		return true
 	}
+	if _, err := exec.LookPath("systemctl"); err != nil {
+		return false
+	}
 	if _, err := os.Stat("/proc/1/comm"); err == nil {
 		filerc, err := os.Open("/proc/1/comm")
 		if err != nil {