[client] Use native windows sock opts to avoid routing loops (#4314)

- Move `util/grpc` and `util/net` to `client` so `internal` packages can be accessed
 - Add methods to return the next best interface after the NetBird interface.
- Use `IP_UNICAST_IF` sock opt to force the outgoing interface for the NetBird `net.Dialer` and `net.ListenerConfig` to avoid routing loops. The interface is picked by the new route lookup method.
- Some refactoring to avoid import cycles
- Old behavior is available through `NB_USE_LEGACY_ROUTING=true` env var

client/net/env.go

@@ -0,0 +1,35 @@
+package net
+
+import (
+	"os"
+	"strconv"
+
+	log "github.com/sirupsen/logrus"
+
+	"github.com/netbirdio/netbird/client/iface/netstack"
+)
+
+const (
+	envDisableCustomRouting = "NB_DISABLE_CUSTOM_ROUTING"
+	envUseLegacyRouting     = "NB_USE_LEGACY_ROUTING"
+)
+
+// CustomRoutingDisabled returns true if custom routing is disabled.
+// This will fall back to the operation mode before the exit node functionality was implemented.
+// In particular exclusion routes won't be set up and all dialers and listeners will use net.Dial and net.Listen, respectively.
+func CustomRoutingDisabled() bool {
+	if netstack.IsEnabled() {
+		return true
+	}
+
+	var customRoutingDisabled bool
+	if val := os.Getenv(envDisableCustomRouting); val != "" {
+		var err error
+		customRoutingDisabled, err = strconv.ParseBool(val)
+		if err != nil {
+			log.Warnf("failed to parse %s: %v", envDisableCustomRouting, err)
+		}
+	}
+
+	return customRoutingDisabled
+}