client/systray: add installer for a freedesktop autostart file (#18767)

Adds freedesktop as an option for installing autostart desktop files for
starting the systray application.

Fixes #18766

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

cmd/tailscale/cli/configure_linux.go

@@ -33,7 +33,7 @@ func systrayConfigCmd() *ffcli.Command {
 		FlagSet: (func() *flag.FlagSet {
 			fs := newFlagSet("systray")
 			fs.StringVar(&systrayArgs.initSystem, "enable-startup", "",
-				"Install startup script for init system. Currently supported systems are [systemd].")
+				"Install startup script for init system. Currently supported systems are [systemd, freedesktop].")
 			return fs
 		})(),
 	}