ipn/ipnlocal, net/dns*, util/cloudenv: specialize DNS config on Google Cloud

This does three things:

* If you're on GCP, it adds a *.internal DNS split route to the
  metadata server, so we never break GCP DNS names. This lets people
  have some Tailscale nodes on GCP and some not (e.g. laptops at home)
  without having to add a Tailnet-wide *.internal DNS route.
  If you already have such a route, though, it won't overwrite it.

* If the 100.100.100.100 DNS forwarder has nowhere to forward to,
  it forwards it to the GCP metadata IP, which forwards to 8.8.8.8.
  This means there are never errNoUpstreams ("upstream nameservers not set")
  errors on GCP due to e.g. mangled /etc/resolv.conf (GCP default VMs
  don't have systemd-resolved, so it's likely a DNS supremacy fight)

* makes the DNS fallback mechanism use the GCP metadata IP as a
  fallback before our hosted HTTP-based fallbacks

I created a default GCP VM from their web wizard. It has no
systemd-resolved.

I then made its /etc/resolv.conf be empty and deleted its GCP
hostnames in /etc/hosts.

I then logged in to a tailnet with no global DNS settings.

With this, tailscaled writes /etc/resolv.conf (direct mode, as no
systemd-resolved) and sets it to 100.100.100.100, which then has
regular DNS via the metadata IP and *.internal DNS via the metadata IP
as well. If the tailnet configures explicit DNS servers, those are used
instead, except for *.internal.

This also adds a new util/cloudenv package based on version/distro
where the cloud type is only detected once. We'll likely expand it in
the future for other clouds, doing variants of this change for other
popular cloud environments.

Fixes #4911

RELNOTES=Google Cloud DNS improvements

Change-Id: I19f3c2075983669b2b2c0f29a548da8de373c7cf
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

go.mod

@@ -3,6 +3,7 @@ module tailscale.com
 go 1.18
 
 require (
+	cloud.google.com/go/compute v1.7.0
 	filippo.io/mkcert v1.4.3
 	github.com/akutz/memconn v0.1.0
 	github.com/alexbrainman/sspi v0.0.0-20210105120005-909beea2cc74
@@ -51,12 +52,12 @@ require (
 	github.com/vishvananda/netlink v1.1.1-0.20211118161826-650dca95af54
 	go4.org/mem v0.0.0-20210711025021-927187094b94
 	golang.org/x/crypto v0.0.0-20220427172511-eb4f295cb31f
-	golang.org/x/net v0.0.0-20220516155154-20f960328961
-	golang.org/x/sync v0.0.0-20220513210516-0976fa681c29
-	golang.org/x/sys v0.0.0-20220513210249-45d2b4557a2a
+	golang.org/x/net v0.0.0-20220607020251-c690dde0001d
+	golang.org/x/sync v0.0.0-20220601150217-0de741cfad7f
+	golang.org/x/sys v0.0.0-20220610221304-9f5ed59c137d
 	golang.org/x/term v0.0.0-20210927222741-03fcf44c2211
 	golang.org/x/time v0.0.0-20211116232009-f0f3c7e86c11
-	golang.org/x/tools v0.1.11-0.20220413170336-afc6aad76eb1
+	golang.org/x/tools v0.1.11
 	golang.zx2c4.com/wireguard v0.0.0-20220317000134-95b48cdb3961
 	golang.zx2c4.com/wireguard/windows v0.4.10
 	gvisor.dev/gvisor v0.0.0-20220407223209-21871174d445
@@ -260,9 +261,8 @@ require (
 	go4.org/intern v0.0.0-20211027215823-ae77deb06f29 // indirect
 	go4.org/unsafe/assume-no-moving-gc v0.0.0-20220617031537-928513b29760 // indirect
 	golang.org/x/exp/typeparams v0.0.0-20220328175248-053ad81199eb // indirect
-	golang.org/x/mod v0.6.0-dev.0.20220106191415-9b9b3d81d5e3 // indirect
+	golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4 // indirect
 	golang.org/x/text v0.3.7 // indirect
-	golang.org/x/xerrors v0.0.0-20220411194840-2f41105eb62f // indirect
 	golang.zx2c4.com/wintun v0.0.0-20211104114900-415007cec224 // indirect
 	google.golang.org/protobuf v1.28.0 // indirect
 	gopkg.in/ini.v1 v1.66.2 // indirect