tun: NetlinkListener: don't send EventDown before sending EventUp

tun: NetlinkListener: don't send EventDown before sending EventUp

This works around a startup race condition when competing with
HackListener, which is trying to do the same job. If HackListener
detects that the tundev is running while there is still an event in the
netlink queue that says it isn't running, then the device receives a
string of events like
	EventUp (HackListener)
	EventDown (NetlinkListener)
	EventUp (NetlinkListener)
Unfortunately, after the first EventDown, the device stops itself,
thinking incorrectly that the administrator has downed its tundev.

The device is ignoring the initial EventDown anyway, so just don't emit
it.

Signed-off-by: Avery Pennarun <apenwarr@tailscale.com>