cmd/tailscaled, ipn/conffile: support ec2 user-data config file

Updates #1412
Updates #1866

Change-Id: I4d08fb233b80c2078b3b28ffc18559baabb4a081
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/depaware.txt

@@ -320,6 +320,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
      💣 tailscale.com/net/tshttpproxy                                from tailscale.com/clientupdate/distsign+
         tailscale.com/net/tstun                                      from tailscale.com/cmd/tailscaled+
         tailscale.com/net/wsconn                                     from tailscale.com/control/controlhttp+
+        tailscale.com/omit                                           from tailscale.com/ipn/conffile
         tailscale.com/paths                                          from tailscale.com/client/tailscale+
      💣 tailscale.com/portlist                                       from tailscale.com/ipn/ipnlocal
         tailscale.com/posture                                        from tailscale.com/ipn/ipnlocal

cmd/tailscaled/tailscaled.go

@@ -118,7 +118,7 @@ var args struct {
 	tunname string
 
 	cleanUp        bool
-	confFile       string
+	confFile       string // empty, file path, or "vm:user-data"
 	debug          string
 	port           uint16
 	statepath      string
@@ -169,7 +169,7 @@ func main() {
 	flag.StringVar(&args.birdSocketPath, "bird-socket", "", "path of the bird unix socket")
 	flag.BoolVar(&printVersion, "version", false, "print version information and exit")
 	flag.BoolVar(&args.disableLogs, "no-logs-no-support", false, "disable log uploads; this also disables any technical support")
-	flag.StringVar(&args.confFile, "config", "", "path to config file")
+	flag.StringVar(&args.confFile, "config", "", "path to config file, or 'vm:user-data' to use the VM's user-data (EC2)")
 
 	if len(os.Args) > 0 && filepath.Base(os.Args[0]) == "tailscale" && beCLI != nil {
 		beCLI()