And use dynamic port numbers in tests, as Linux on GitHub Actions and
Windows in general have things running on these ports.
Co-Author: Julian Knodt <julianknodt@gmail.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
PCP handles external IPs by allowing the client to specify them in the packet, which is more
explicit than requiring 2 packets from PMP, so allow for future changes to add it in easily.
Signed-off-by: julianknodt <julianknodt@gmail.com>
