feat(dns): use system nameservers from /etc/resolv.conf as fallback

When no forwarder or dns metadata is configured, the DNS handler now
reads system nameservers from /etc/resolv.conf (Unix) before falling
back to the hardcoded 127.0.0.1:53 default. This makes the DNS proxy
work out-of-the-box without requiring explicit DNS server configuration.

Fixes go-gost/gost#89

handler/dns/resolv_windows.go

@@ -0,0 +1,8 @@
+package dns
+
+// systemNameservers returns nil on Windows because reading the system DNS
+// configuration requires the Windows IP Helper API. Callers should fall
+// back to the hardcoded default when this returns nil.
+func systemNameservers() []string {
+	return nil
+}