cmd/cli: implement valid interfaces map for all systems

Previously, a valid interfaces map is only meaningful on Windows and
Darwin, where ctrld needs to set DNS for all physical interfaces.

With new network monitor, the valid interfaces is used for checking new
changes, thus we have to implement the valid interfaces map for all
systems.

 - On Linux, just retrieving all non-virtual interfaces.
 - On others, fallback to use default route interface only.

cmd/cli/net_others.go

@@ -1,11 +1,22 @@
-//go:build !darwin && !windows
+//go:build !darwin && !windows && !linux
 
 package cli
 
-import "net"
+import (
+	"net"
+
+	"tailscale.com/net/netmon"
+)
 
 func patchNetIfaceName(iface *net.Interface) (bool, error) { return true, nil }
 
 func validInterface(iface *net.Interface, validIfacesMap map[string]struct{}) bool { return true }
 
-func validInterfacesMap() map[string]struct{} { return nil }
+// validInterfacesMap returns a set containing only default route interfaces.
+func validInterfacesMap() map[string]struct{} {
+	defaultRoute, err := netmon.DefaultRoute()
+	if err != nil {
+		return nil
+	}
+	return map[string]struct{}{defaultRoute.InterfaceName: {}}
+}