global: remove TODO name graffiti

Googlers have a habit of graffiting their name in TODO items that then
are never addressed, and other people won't go near those because
they're marked territory of another animal. I've been gradually cleaning
these up as I see them, but this commit just goes all the way and
removes the remaining stragglers.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tun/tuntest/tuntest.go

@@ -113,7 +113,7 @@ func (t *chTun) File() *os.File { return nil }
 func (t *chTun) Read(data []byte, offset int) (int, error) {
 	select {
 	case <-t.c.closed:
-		return 0, io.EOF // TODO(crawshaw): what is the correct error value?
+		return 0, os.ErrClosed
 	case msg := <-t.c.Outbound:
 		return copy(data[offset:], msg), nil
 	}
@@ -130,7 +130,7 @@ func (t *chTun) Write(data []byte, offset int) (int, error) {
 	copy(msg, data[offset:])
 	select {
 	case <-t.c.closed:
-		return 0, io.EOF // TODO(crawshaw): what is the correct error value?
+		return 0, os.ErrClosed
 	case t.c.Inbound <- msg:
 		return len(data) - offset, nil
 	}