[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/net_linux.go

@@ -0,0 +1,55 @@
+//go:build !android
+
+package net
+
+import (
+	"fmt"
+	"syscall"
+)
+
+// SetSocketMark sets the SO_MARK option on the given socket connection
+func SetSocketMark(conn syscall.Conn) error {
+	if !AdvancedRouting() {
+		return nil
+	}
+
+	sysconn, err := conn.SyscallConn()
+	if err != nil {
+		return fmt.Errorf("get raw conn: %w", err)
+	}
+
+	return setRawSocketMark(sysconn)
+}
+
+// SetSocketOpt sets the SO_MARK option on the given file descriptor
+func SetSocketOpt(fd int) error {
+	if !AdvancedRouting() {
+		return nil
+	}
+
+	return setSocketOptInt(fd)
+}
+
+func setRawSocketMark(conn syscall.RawConn) error {
+	var setErr error
+
+	err := conn.Control(func(fd uintptr) {
+		if !AdvancedRouting() {
+			return
+		}
+		setErr = setSocketOptInt(int(fd))
+	})
+	if err != nil {
+		return fmt.Errorf("control: %w", err)
+	}
+
+	if setErr != nil {
+		return fmt.Errorf("set SO_MARK: %w", setErr)
+	}
+
+	return nil
+}
+
+func setSocketOptInt(fd int) error {
+	return syscall.SetsockoptInt(fd, syscall.SOL_SOCKET, syscall.SO_MARK, ControlPlaneMark)
+}