Linux legacy routing (#1774)

* Add Linux legacy routing if ip rule functionality is not available

* Ignore exclusion route errors if host has no route

* Exclude iOS from route manager

* Also retrieve IPv6 routes

* Ignore loopback addresses not being in the main table

* Ignore "not supported" errors on cleanup

* Fix regression in ListenUDP not using fwmarks

client/internal/routemanager/routemanager.go

@@ -1,8 +1,9 @@
-//go:build !android
+//go:build !android && !ios
 
 package routemanager
 
 import (
+	"errors"
 	"fmt"
 	"net/netip"
 	"sync"
@@ -53,6 +54,9 @@ func (rm *RouteManager) AddRouteRef(connID nbnet.ConnectionID, prefix netip.Pref
 	if ref.count == 0 {
 		log.Debugf("Adding route for prefix %s", prefix)
 		nexthop, intf, err := rm.addRoute(prefix)
+		if errors.Is(err, errRouteNotFound) {
+			return nil
+		}
 		if err != nil {
 			return fmt.Errorf("failed to add route for prefix %s: %w", prefix, err)
 		}