Add systemd.SdBooted()

This is a conversion of sd_booted() from libsystemd to go and checks
if the system was booted with systemd.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)

systemd/booted.go

@@ -0,0 +1,15 @@
+package systemd
+
+import (
+	"os"
+)
+
+// Conversion to Go of systemd's sd_booted()
+func SdBooted() bool {
+	s, err := os.Stat("/run/systemd/system")
+	if err != nil {
+		return false
+	}
+
+	return s.IsDir()
+}