system: move sd_booted implementation to os abstraction

This implements lazy loading of the systemd marker (they are not accessed in init and shim) and ensures consistent behaviour when running with a stub.

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

main.go

@@ -30,7 +30,7 @@ func main() {
 	flag.Parse()
 	fmsg.SetVerbose(flagVerbose)
 
-	if internal.SdBootedV {
+	if os.SdBooted() {
 		fmsg.VPrintln("system booted with systemd as init system")
 	}