will be used in the next patch

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

note: man for /proc/uptime says that uptime is in seconds and for now
the format is "seconds.centiseconds", where ecentiseconds is 2 digits

note: now uptime is in csec but I prefer saving it in usec, that allows
us to be reuse these image field when/if we have more accurate value.

v8: add length specifier to parse only centiseconds
v9: put uptime to u_int64_t directly, define CSEC_PER_SEC
v10: switch to uint64_t from u_int64_t, comment about usec in image
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>