wgengine/netstack: include DNS.ExtraRecords in DNSMap

So SOCKS5 dialer can dial HTTPS cert names, for instance.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
(cherry picked from commit 29a8fb45d3)

wgengine/netstack/netstack.go

@@ -206,6 +206,16 @@ func DNSMapFromNetworkMap(nm *netmap.NetworkMap) DNSMap {
 			break
 		}
 	}
+	for _, rec := range nm.DNS.ExtraRecords {
+		if rec.Type != "" {
+			continue
+		}
+		ip, err := netaddr.ParseIP(rec.Value)
+		if err != nil {
+			continue
+		}
+		ret[strings.TrimRight(rec.Name, ".")] = ip
+	}
 	return ret
 }