wgengine/monitor: start moving interface state accessor into monitor

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/monitor/monitor.go

@@ -11,6 +11,7 @@ import (
 	"sync"
 	"time"
 
+	"tailscale.com/net/interfaces"
 	"tailscale.com/types/logger"
 )
 
@@ -72,6 +73,16 @@ func New(logf logger.Logf) (*Mon, error) {
 	}, nil
 }
 
+// InterfaceState returns the state of the machine's network interfaces,
+// without any Tailscale ones.
+func (m *Mon) InterfaceState() (*interfaces.State, error) {
+	s, err := interfaces.GetState()
+	if s != nil {
+		s.RemoveTailscaleInterfaces()
+	}
+	return s, err
+}
+
 // RegisterChangeCallback adds callback to the set of parties to be
 // notified (in their own goroutine) when the network state changes.
 // To remove this callback, call unregister (or close the monitor).