cmd/tailscale, net/portmapper: add --log-http option to "debug portmap"

This option allows logging the raw HTTP requests and responses that the
portmapper Client makes when using UPnP. This can be extremely helpful
when debugging strange UPnP issues with users' devices, and might allow
us to avoid having to instruct users to perform a packet capture.

Updates #8992

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I2c3cf6930b09717028deaff31738484cc9b008e4

net/portmapper/portmapper.go

@@ -36,6 +36,11 @@ type DebugKnobs struct {
 	// to its logger.
 	VerboseLogs bool
 
+	// LogHTTP tells the Client to print the raw HTTP logs (from UPnP) to
+	// its logger. This is useful when debugging buggy UPnP
+	// implementations.
+	LogHTTP bool
+
 	// Disable* disables a specific service from mapping.
 	DisableUPnP bool
 	DisablePMP  bool