[CRIU] [RFC] Introduce v1.1 image format

[Pavel Emelyanov]

Hi,

Right now we have too many criu-specific file types -- effectively
every new image file we add is a new "type", we invent a new magic
number for it and put only it into the header.

One of the difficulty with this approach is that we can't nicely
teach the "file" utility to spot CRIU-specific files. Too many
magic should be added and we would have to add new magic numbers
for every new image type we introduce.

So the proposal is to turn all image files into one format. For
compatibility reasons the inventory file will have to remain
intact. But for the rest the common 4-bytes magic is introduced.
The old-style magic follows one.

There are two more exceptions from this rule -- stats and irmap
cache files. These two are not images, so these are not changed.
(Maybe it's worth joining them under "criu service file" magic?)

Older versions of CRIU will not be able to restore from v1.1, but
that's OK. If we will need it, converting formats would be quite
easy. Newer CRIU will still understand v1 format, so no crit-based
conversion required.

-- Pavel