net/portmapper: fix UPnP probing

[bradfitz]

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 use the Location header to find the rootDesc.xml, rather than hard-coding the URL + port.

And remove the three goroutines doing redundant & racing rootDesc.xml fetches.

And use netns so it works while using exit nodes.

Works against pfSense now.

Updates #2109

[bradfitz]

I'm now getting, against pfSense:

root@tailtest:~# ./tailscaled debug -portmap
2021/08/02 22:07:16 portmapper: unexpected PMP probe response: {OpCode:128 ResultCode:3 SecondsSinceEpoch:8671 MappingValidSeconds:0 InternalPort:0 ExternalPort:0 PublicAddr:0.0.0.0}
2021/08/02 22:07:16 portmapper: Got PCP response: epoch: 8671
2021/08/02 22:07:16 UPnP reply from 192.168.1.1:2189: HTTP/1.1 200 OK
CACHE-CONTROL: max-age=120
ST: urn:schemas-upnp-org:device:InternetGatewayDevice:1
USN: uuid:bee7052b-49e8-3597-b545-55a1e38ac11::urn:schemas-upnp-org:device:InternetGatewayDevice:1
EXT:
SERVER: FreeBSD/12.2-STABLE UPnP/1.1 MiniUPnPd/2.2.1
LOCATION: http://192.168.1.1:2189/rootDesc.xml
OPT: "http://schemas.upnp.org/upnp/1/0/"; ns=01
01-NLS: 1627958564
BOOTID.UPNP.ORG: 1627958564
CONFIGID.UPNP.ORG: 1337

2021/08/02 22:07:16 Probe: {PCP:true PMP:false UPnP:true}
2021/08/02 22:07:16 no mapping
2021/08/02 22:07:16 Getting root desc: http://192.168.1.1:2189/rootDesc.xml
2021/08/02 22:07:16 XXX meth1: [0xc000315200]
2021/08/02 22:07:16 XXX client = &{{0xc000336000 0xc000212000 http://192.168.1.1:2189/rootDesc.xml Service ID urn:upnp-org:serviceId:WANIPConn1 : urn:schemas-upnp-org:service:WANIPConnection:1}}
2021/08/02 22:07:16 XXX meth2: error requesting root device details from "http://192.168.1.1:2189/rootDesc.xml": Get "http://192.168.1.1:2189/rootDesc.xml": context canceled
2021/08/02 22:07:16 XXX meth3: error requesting root device details from "http://192.168.1.1:2189/rootDesc.xml": Get "http://192.168.1.1:2189/rootDesc.xml": context canceled
2021/08/02 22:07:16 XXX addAny = SOAP fault: 

[crawshaw]

woo tests