ipn/ipnlocal, net/dns*, util/cloudenv: add AWS DNS support

And remove the GCP special-casing from ipn/ipnlocal; do it only in the
forwarder for *.internal.

Fixes #4980
Fixes #4981

Change-Id: I5c481e96d91f3d51d274a80fbd37c38f16dfa5cb
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/resolver/tsdns.go

@@ -31,6 +31,7 @@ import (
 	"tailscale.com/types/dnstype"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/clientmetric"
+	"tailscale.com/util/cloudenv"
 	"tailscale.com/util/dnsname"
 	"tailscale.com/wgengine/monitor"
 )
@@ -97,6 +98,9 @@ func (c *Config) WriteToBufioWriter(w *bufio.Writer) {
 	if arpa > 0 {
 		fmt.Fprintf(w, "+%darpa", arpa)
 	}
+	if c := cloudenv.Get(); c != "" {
+		fmt.Fprintf(w, ", cloud=%q", string(c))
+	}
 	w.WriteString("}")
 }