net/dns: skip DNS base config when using userspace networking (#18355)

When tailscaled gets started with userspace networking, it won't
modify your system's network configuration. For this, it creates
a noopManager for DNS management. noopManager correctly observes
that there's no real OS DNS to send queries to. This leads to we
completely dropping any DNS internal resolution from `dns query`

This change alters this so that even without a base config we'll
still allow the internal resolver to handle internal DNS queries

Fixes #18354

Signed-off-by: Fernando Serboncini <fserb@tailscale.com>

net/dns/noop.go

@@ -15,3 +15,8 @@ func (m noopManager) GetBaseConfig() (OSConfig, error) {
 func NewNoopManager() (noopManager, error) {
 	return noopManager{}, nil
 }
+
+func isNoopManager(c OSConfigurator) bool {
+	_, ok := c.(noopManager)
+	return ok
+}