Script to produce Prometheus-style `.prom` files from `pfctl(8)` output.
By default places output in `/var/tmp/node_exporter`, for the benefit of `node_exporter`.

Currently only collects data from running jails (not the host itself).

## Assumptions
The script first checks for the existence of `/dev/pf`.

Jails with a `/dev/pf` have paths either of the form
  /x/y
or
  /x/y/root

Both cases are checked, and if `/sbin/pfctl` and `/libexec/ld-elf.so.1` are found, they are used as-is.

If not, the former pattern is expected to be the parent of a wrapped jail, with no libraries or binaries in the regular paths.
The inner jail is expected to have a path prefix of /root relative to the parent; In such cases, the script sets `LD_LIBRARY_PATH`
to `/root/lib` and invokes `/root/libexec/ld-elf.so.1` to execute `/root/sbin/pfctl`.

## Work to be done
- Be more flexible about jail path layouts
- Support collecting pf stats for the host
- Collect per-rule statistics
- Collect per-table statistics