[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/protectsocket_android.go

@@ -0,0 +1,47 @@
+package net
+
+import (
+	"fmt"
+	"sync"
+	"syscall"
+
+	"github.com/netbirdio/netbird/client/iface/netstack"
+)
+
+var (
+	androidProtectSocketLock sync.Mutex
+	androidProtectSocket     func(fd int32) bool
+)
+
+func SetAndroidProtectSocketFn(fn func(fd int32) bool) {
+	androidProtectSocketLock.Lock()
+	androidProtectSocket = fn
+	androidProtectSocketLock.Unlock()
+}
+
+// ControlProtectSocket is a Control function that sets the fwmark on the socket
+func ControlProtectSocket(_, _ string, c syscall.RawConn) error {
+	if netstack.IsEnabled() {
+		return nil
+	}
+	var aErr error
+	err := c.Control(func(fd uintptr) {
+		androidProtectSocketLock.Lock()
+		defer androidProtectSocketLock.Unlock()
+
+		if androidProtectSocket == nil {
+			aErr = fmt.Errorf("socket protection function not set")
+			return
+		}
+
+		if !androidProtectSocket(int32(fd)) {
+			aErr = fmt.Errorf("failed to protect socket via Android")
+		}
+	})
+
+	if err != nil {
+		return err
+	}
+
+	return aErr
+}