util: Ability to ignore errno when opening proc

When run from regular user criu will get EACCES/EPERM from
opening proc, but in some situations criu will now how to
deal with it. So this patch makes it possible not to print
error message in logs for such cases.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Looks-good-to-me: Andrew Vagin <avagin@virtuozzo.com>