files: Add fake fle flag and close such fles after restore files

Allow to mark some fles as "fake" and close them, when they are not needed.

v2: Cache fake file in separate list, when they are restored,
to do not iterate over files list again.

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