net/dns/resolver: add live reconfig, plumb through to ipnlocal.

The resolver still only supports a single upstream config, and
ipn/wgengine still have to split up the DNS config, but this moves
closer to unifying the DNS configs.

As a handy side-effect of the refactor, IPv6 MagicDNS records exist
now.

Signed-off-by: David Anderson <danderson@tailscale.com>

wgengine/userspace_test.go

@@ -108,7 +108,7 @@ func TestUserspaceEngineReconfig(t *testing.T) {
 			},
 		}
 
-		err = e.Reconfig(cfg, routerCfg)
+		err = e.Reconfig(cfg, routerCfg, nil, nil)
 		if err != nil {
 			t.Fatal(err)
 		}