portmapper.go was introduced for UPnP in commit
8ed43f1f2a.
This also adds a unit test which exercises the incomplete
natlab server implementation of these portmapping protocols.
Updates #21713
Change-Id: I33b61ccbe8c1d8d99d56aa357ae24777be3f5739
Signed-off-by: Francois Marier <francois@tailscale.com>