coredump: introduce is_journald_crash() and is_pid1_crash() helpers

coredump: introduce is_journald_crash() and is_pid1_crash() helpers

We check these a number of times, hence let's unify these checks here.
This also allows us to make the PID 1 check more elaborate as we can
check both the PID and the cgroup. Checking the PID has the benefit that
we'll also cover cases where PID 1 might still be in the root cgroup, and
the cgroup check has the benefit that we also cover crashes in forked
off crasher processes (the way we actually do it in systemd)