We were copying 12 out of the 16 bytes which meant that
the 1:1 NAT required would only work if the last 4 bytes
happened to match between the new and old address, something
that our tests accidentally had. Fix it by copying the full
16 bytes and make the tests also verify the addr and use rand
addresses.
Updates #9511
Signed-off-by: Maisem Ali <maisem@tailscale.com>
The current structure meant that we were embedding netstack in
the tailscale CLI and in the GUIs. This removes that by isolating
the checksum munging to a different pkg which is only called from
`net/tstun`.
Fixes#9756
Signed-off-by: Maisem Ali <maisem@tailscale.com>