And refactor some of vnet.go for testability.
The only behavioral change (with a new test) is that ethernet
broadcasts no longer get sent back to the sender.
Updates #13038
Change-Id: Ic2e7e7d6d8805b7b7f2b5c52c2c5ba97101cef14
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
