quadlet: Add --user option

Normally quadlet picks up whether to behave like a user or system
instance based on the binary name, but for the tests we want the
ability to pass `--user` to modify this using a single binary.

Signed-off-by: Alexander Larsson <alexl@redhat.com>

cmd/quadlet/main.go

@@ -358,4 +358,5 @@ func main() {
 func init() {
 	flag.BoolVar(&verboseFlag, "v", false, "Print debug information")
 	flag.BoolVar(&noKmsgFlag, "no-kmsg-log", false, "Don't log to kmsg")
+	flag.BoolVar(&isUser, "user", false, "Run as systemd user")
 }