Archive /var/run/snabb directory in case of crash

[wingo]

The /var/run/snabb/PID directory contains counter data that can be useful in post-mortem debugging. With raptorjit, we'll also have the auditlog as well; and in the future we'll also have historical counter values (https://github.com/Igalia/snabb/pull/1088), if enabled.

If a process crashes, we should arrange for the supervisor to archive the /var/run/snabb/PID directory somewhere; perhaps /var/run/snabb/crashes/XXXXXX or something.