cmd/tailscale{,d}: move debug subcommand to tailscaled

Work on reducing the size of the tailscale binary, which is
currently pulling in most of the same code as tailscaled.

Updates #1181

cmd/tailscale/cli/cli.go

@@ -68,11 +68,6 @@ change in the future.
 		Exec:    func(context.Context, []string) error { return flag.ErrHelp },
 	}
 
-	// Don't advertise the debug command, but it exists.
-	if strSliceContains(args, "debug") {
-		rootCmd.Subcommands = append(rootCmd.Subcommands, debugCmd)
-	}
-
 	if err := rootCmd.Parse(args); err != nil {
 		return err
 	}
@@ -134,12 +129,3 @@ func pump(ctx context.Context, bc *ipn.BackendClient, conn net.Conn) {
 		bc.GotNotifyMsg(msg)
 	}
 }
-
-func strSliceContains(ss []string, s string) bool {
-	for _, v := range ss {
-		if v == s {
-			return true
-		}
-	}
-	return false
-}