cgrulesengd: Do not ignore changes of short-lived processes

cgrulesengd: Do not ignore changes of short-lived processes

When a double-forking daemon spawns the shortlived forking process and
we fail to classify it in time, the child does not inherit (the
intended) cgroup membership.

We could process all children after receiving PROC_EVENT_FORK to remedy
this. But since we already have the timestamp logic introduced in

    8953fc07c049 ("Changelog v2:  * Use clock_gettime(2) for getting
    timestamp since a system boot.  * Change parent_info's memory to
    dynamic allocation.")

and it may be too much work for all fork(2) calls, we extend the usage
of parent_info by assuming the parent would have changed its cgroup
membership by our actions even if it terminated quickly.

v2: Handle non-existent /proc/$PID/tasks as short-lived process too
    Use cgroup_get_last_errno() helper

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Dhaval Giani <dhaval.giani@gmail.com>