systemd-coredump: add COREDUMP_CODE (#42019)

Add COREDUMP_CODE to the fields captured by systemd-coredump. This makes
it possible for system administrators to filter coredumps based on si_code,
which describes the reason why a given signal was sent.

For example, to find processes killed due to invalid permissions
(SEGV_ACCERR):

$ journalctl COREDUMP_SIGNAL=11 COREDUMP_CODE=2

I've decided to add the value of si_code to the 'Signal: ' line of
coredumpctl info:
    Signal: 11 (SEGV) si_code: SEGV_ACCERR