add hard restart for updates on *nix

reincarnates app.Restart() removed in
bbb0568cc4 as app.HardRestart().

restart_windows.go

@@ -0,0 +1,7 @@
+package main
+
+import "fmt"
+
+func (app *appContext) HardRestart() error {
+	return fmt.Errorf("hard restarts not available on windows")
+}