wgengine: don't lose filter state on filter reconfig.

We were abandoning the UDP port LRU every time we got a new packet
filter from tailcontrol, which caused return packets to suddenly stop
arriving.

wgengine/wgengine.go

@@ -101,6 +101,9 @@ type Engine interface {
 	// sends an updated network map.
 	Reconfig(cfg *wgcfg.Config, dnsDomains []string) error
 
+	// GetFilter returns the current packet filter, if any
+	GetFilter() *filter.Filter
+
 	// SetFilter updates the packet filter.
 	SetFilter(*filter.Filter)