net/tstun: split TUN events channel into up/down and MTU

We had a long-standing bug in which our TUN events channel
was being received from simultaneously in two places.

The first is wireguard-go.

At wgengine/userspace.go:366, we pass e.tundev to wireguard-go,
which starts a goroutine (RoutineTUNEventReader)
that receives from that channel and uses events to adjust the MTU
and bring the device up/down.

At wgengine/userspace.go:374, we launch a goroutine that
receives from e.tundev, logs MTU changes, and triggers
state updates when up/down changes occur.

Events were getting delivered haphazardly between the two of them.

We don't really want wireguard-go to receive the up/down events;
we control the state of the device explicitly by calling device.Up.
And the userspace.go loop MTU logging duplicates logging that
wireguard-go does when it received MTU updates.

So this change splits the single TUN events channel into up/down
and other (aka MTU), and sends them to the parties that ought
to receive them.

I'm actually a bit surprised that this hasn't caused more visible trouble.
If a down event went to wireguard-go but the subsequent up event
went to userspace.go, we could end up with the wireguard-go device disappearing.

I believe that this may also (somewhat accidentally) be a fix for #1790.

Signed-off-by: Josh Bleecher Snyder <josharian@gmail.com>

wgengine/userspace.go

@@ -373,11 +373,7 @@ func NewUserspaceEngine(logf logger.Logf, conf Config) (_ Engine, reterr error)
 
 	go func() {
 		up := false
-		for event := range e.tundev.Events() {
-			if event&tun.EventMTUUpdate != 0 {
-				mtu, err := e.tundev.MTU()
-				e.logf("external route MTU: %d (%v)", mtu, err)
-			}
+		for event := range e.tundev.EventsUpDown() {
 			if event&tun.EventUp != 0 && !up {
 				e.logf("external route: up")
 				e.RequestStatus()