control/controlclient,util/execqueue: extract execqueue into a package

This is a useful primitive for asynchronous execution of ordered work I
want to use in another change.

Updates tailscale/corp#16833
Signed-off-by: James Tucker <james@tailscale.com>

util/execqueue/execqueue_test.go

@@ -0,0 +1,22 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package execqueue
+
+import (
+	"context"
+	"sync/atomic"
+	"testing"
+)
+
+func TestExecQueue(t *testing.T) {
+	ctx := context.Background()
+	var n atomic.Int32
+	q := &ExecQueue{}
+	defer q.Shutdown()
+	q.Add(func() { n.Add(1) })
+	q.Wait(ctx)
+	if got := n.Load(); got != 1 {
+		t.Errorf("n=%d; want 1", got)
+	}
+}