Add Support for SCM_CREDENTIALS / SCM_PIDFD and Friends

Currently, CRIU cannot checkpoint or restore systemd-managed containers if there are SCM_CREDENTIALS messages waiting in a Unix socket queue. Because systemd uses these credentials for almost all service communications, any in-flight message causes CRIU to fail. We aim to solve this problem by adding support for SCM_CREDENTIALS and the newer SCM_PIDFD to CRIU’s socket queue logic. I will implement PID remapping to ensure credentials remain valid after restore and add support for modern socket options like SO_PASSPIDFD. The final deliverables will include core code updates to the socket subsystem and extend image formats.