coredump: restore compatibility with older patterns

coredump: restore compatibility with older patterns

This was broken in f45b8015513d38ee5f7cc361db9c5b88c9aae704. Unfortunately
the review does not talk about backward compatibility at all. There are
two places where it matters:
- During upgrades, the replacement of kernel.core_pattern is asynchronous.
  For example, during rpm upgrades, it would be updated a post-transaction
  file trigger. In other scenarios, the update might only happen after
  reboot. We have a potentially long window where the old pattern is in
  place. We need to capture coredumps during upgrades too.
- With --backtrace. The interface of --backtrace, in hindsight, is not
  great. But there are users of --backtrace which were written to use
  a specific set of arguments, and we can't just break compatiblity.
  One example is systemd-coredump-python, but there are also reports of
  users using --backtrace to generate coredump logs.

Thus, we require the original set of args, and will use the additional args if
found.

A test is added to verify that --backtrace works with and without the optional
args.

(cherry picked from commit ded0aac389e647d35bce7ec4a48e718d77c0435b)