TUN-4063: Cleanup dependencies between packages.

- Move packages the provide generic functionality (such as config) from `cmd` subtree to top level.
- Remove all dependencies on `cmd` subtree from top level packages.
- Consolidate all code dealing with token generation and transfer to a single cohesive package.

token/token_test.go

@@ -0,0 +1,54 @@
+//+build linux
+
+package token
+
+import (
+	"os"
+	"syscall"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSignalHandler(t *testing.T) {
+	sigHandler := signalHandler{signals: []os.Signal{syscall.SIGUSR1}}
+	handlerRan := false
+	done := make(chan struct{})
+	timer := time.NewTimer(time.Second)
+	sigHandler.register(func(){
+		handlerRan = true
+		done <- struct{}{}
+	})
+
+	p, err := os.FindProcess(os.Getpid())
+	require.Nil(t, err)
+	p.Signal(syscall.SIGUSR1)
+
+	// Blocks for up to one second to make sure the handler callback runs before the assert.
+	select {
+		case <- done:
+			assert.True(t, handlerRan)
+		case <- timer.C:
+			t.Fail()
+	}
+	sigHandler.deregister()
+}
+
+func TestSignalHandlerClose(t *testing.T) {
+	sigHandler := signalHandler{signals: []os.Signal{syscall.SIGUSR1}}
+	done := make(chan struct{})
+	timer := time.NewTimer(time.Second)
+	sigHandler.register(func(){done <- struct{}{}})
+	sigHandler.deregister()
+
+	p, err := os.FindProcess(os.Getpid())
+	require.Nil(t, err)
+	p.Signal(syscall.SIGUSR1)
+	select {
+	case <- done:
+		t.Fail()
+	case <- timer.C:
+	}
+}