criu: introduce LUO image backend

[Thesola10]

This patch adds Live Update Orchestrator as a storage backend for CRIU images.

What this PR does

It adds two flags, --luo-session and --luo-session-fd, which specify a LUO session to save CRIU images into.

If CRIU owns the session file descriptor, that is the flag --luo-session is used, CRIU will remain open in order for the session file descriptor to be preserved when kexec is called.

How it works

A new module, luo.c, has been introduced to manage LUO session file descriptors as a storage medium, by preserving memfds as is recommended in the kernel documentation.

Dump side:

• If --luo-session is specified, a new LUO session is allocated with the name given as argument.
• For each image, a new memfd is opened and returned as the target file descriptor
• Once the image is closed, a token is generated from the image type and its numerical parameter (usually the PID or a small index) for subsequent retrieval.
• Again, if --luo-session is specified, CRIU hangs on a no-op in order to keep the session FD open until kexec

Restore side:

• If --luo-session is specified, CRIU will query /dev/liveupdate for a session with that name.
• When opening an image, the token is derived from the requested file name as above, and the corresponding file descriptor is returned.
• If --luo-session is specified, the LUO session is finalized and its resources are freed.

Implementation notes

• The UAPI header for Live Update Orchestrator has been added into criu/include/linux/liveupdate.h
• --luo-session-fd is intended for an externally managed file descriptor (ideally systemd?) and CRIU will not attempt to close or preserve it.

TODO:

• Short out the current file descriptor serializer in favor of natively preserving file descriptors as-is in the LUO session
• Save memory pages with zero-copy into the target memfd
• Introduce an end-to-end test for proper image persistence on LUO-enabled systems