Prior to Tailscale 1.12 it detected UPnP on any port.
Starting with Tailscale 1.11.x, it stopped detecting UPnP on all ports.
Then start plumbing its discovered Location header port number to the
code that was assuming port 5000.
Fixes#2109
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
