image: Introduce files.img and file_entry

There are two goals of this merge. First is to reduce the amount
of image files we generate and scan on restore. The latter is
more importaint, as even if we have no weird stuff like signalfd,
we still try to open this file. So after the merge we try to
open ~15 image files (out of ~30) less %) which is nice.

The 2nd goal is to simplify the C/R support for SCM messages.
This becomes possible with the fact, that all files we have can
be distinguished by their ID only, w/o type. This, in turn,
makes image layout for SCMs much simpler.

Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>