wgengine: flush DNS cache after major link change.

Windows has a public dns.Flush used in router_windows.go.
However that won't work for platforms like Linux, where
we need a different flush mechanism for resolved versus
other implementations.

We're instead adding a FlushCaches method to the dns Manager,
which can be made to work on all platforms as needed.

Fixes https://github.com/tailscale/tailscale/issues/2132

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

net/dns/manager.go

@@ -212,6 +212,10 @@ func (m *Manager) Down() error {
 	return nil
 }
 
+func (m *Manager) FlushCaches() error {
+	return flushCaches()
+}
+
 // Cleanup restores the system DNS configuration to its original state
 // in case the Tailscale daemon terminated without closing the router.
 // No other state needs to be instantiated before this runs.