sd-event: re-check new epoll events when a child event is queued

sd-event: re-check new epoll events when a child event is queued

Previously, when a process outputs something and exit just after
epoll_wait() but before process_child(), then the IO event is ignored
even if the IO event has higher priority. See #18190.

This can be solved by checking epoll event again after process_child().

However, there exists a possibility that another process outputs and
exits just after process_child() but before the second epoll_wait().
When the IO event has lower priority than the child event, still IO
event is processed.

So, this makes new epoll events and child events are checked in a loop
until no new event is detected. To prevent an infinite loop, the number
of maximum trial is set to 10.

Fixes #18190.