We want to use a simple fact: If we have an alive process in a pstree we
want to dump, and a starttime of that process is less than pre-dump's
timestamp (taken while all processes were freezed), then these exact
process existed (100% sure) at the time of these pre-dump and the
process' memory was dumped in images.

So save inventory image on pre-dump and put there an uptime.

https://jira.sw.ru/browse/PSBM-67502

v9: improve comment, put uptime to ivnentory image as 1) where is no
stats in parent images directory if --work-dir option is set to
something different then images directory, 2) stats-dump is not an image
and it is a bad practice to put there data required for restoring.
v10:s/u_int64_t/uint64_t/
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>