shutdown: correctly wait for processes we killed in the killall spree

Previously we simply counted how many processes we killed and expected
as many waitpid() calls to succeed. That however is incorrect to do.

As we might kill processes that are not our immediate children, and as
there might be left-over processes in the waitpid() queue from earlier
the we might get more ore less waitpid() events that we expect.

Hence: keep precise track of the processes we kill, remove the ones we
get waitpid() for, and after each time we get SIGCHLD check if all
others still exist. We use getpgid() to check if a PID still exists.

This should fix issues with journald not setting journal files offline
correctly on shutdown, because we'd too quickly proceed from SIGTERM to
SIGKILL because some left-over process was in our waitpid() queue.

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