client/systray: move cmd/systray to client/systray

Updates #1708

Change-Id: Ia101a4a3005adb9118051b3416f5a64a4a45987d
Signed-off-by: Will Norris <will@tailscale.com>

client/systray/logo.go

@@ -3,7 +3,7 @@
 
 //go:build cgo || !darwin
 
-package main
+package systray
 
 import (
 	"bytes"

client/systray/systray.go

@@ -3,8 +3,8 @@
 
 //go:build cgo || !darwin
 
-// The systray command is a minimal Tailscale systray application for Linux.
+// Package systray provides a minimal Tailscale systray application.
-package main
+package systray
 
 import (
 	"context"
@@ -44,8 +44,8 @@ var (
 	hideMullvadCities bool
 )
 
-func main() {
+// Run starts the systray menu and blocks until the menu exits.
-	menu := new(Menu)
+func (menu *Menu) Run() {
 	menu.updateState()
 
 	// exit cleanly on SIGINT and SIGTERM