coredump: process special crashes in an (almost) normal way

      We would only log a terse message when pid1 or systemd-journald crashed.
It seems better to reuse the normal code paths as much as possible,
with the following differences:
- if pid1 crashes, we cannot launch the helper, so we don't analyze the
 coredump, just write it to file directly from the helper invoked by the
 kernel;
- if journald crashes, we can produce the backtrace, but we don't log full
  structured messages.

With comparison to previous code, advantages are:
- we go through most of the steps, so for example vacuuming is performed,
- we gather and log more data. In particular for journald and pid1 crashes we
  generate a backtrace, and for pid1 crashes we record the metadata (fdinfo,
  maps, etc.),
- coredumpctl shows pid1 crashes.

A disavantage (inefficiency) is that we gather metadata for journald crashes
which is then ignored because _TRANSPORT=kernel does not support structued
messages.

Messages for the syste