windows/svc/mgr: add Mgr.ListServices

Add API to list services installed on the system.

Fixes golang/go#20596

Change-Id: Ifa2f20ef15ccb962bd21d03788ce931dd45f2630
Reviewed-on: https://go-review.googlesource.com/45711
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

windows/svc/mgr/mgr_test.go

@@ -150,5 +150,20 @@ func TestMyService(t *testing.T) {
 
 	testConfig(t, s, c)
 
+	svcnames, err := m.ListServices()
+	if err != nil {
+		t.Fatalf("ListServices failed: %v", err)
+	}
+	var myserviceIsInstalled bool
+	for _, sn := range svcnames {
+		if sn == name {
+			myserviceIsInstalled = true
+			break
+		}
+	}
+	if !myserviceIsInstalled {
+		t.Errorf("ListServices failed to find %q service", name)
+	}
+
 	remove(t, s)
 }