feat: report listening sockets

get_listening_sockets() in the shape of napalm-device-types'
ListeningSocketsMixin, whose ss/cgroup reading is Linux's, and with its
rule: read as root first, and without root when that brings nothing back,
which the reading reports as `attributed: False`.

- FreeBSD: `sockstat -46lq -P tcp,udp`, which sees every socket either way.
- OpenBSD: `fstat -n` as root (the sockets nothing is connected to; port 0
  is unbound), `netstat -an` without root, as fstat shows a user only
  their own processes; those sockets come without a process.
- Addresses as ss prints them: `*` becomes 0.0.0.0 or :: by family, IPv6
  without brackets, a zone (`fe80::1%lo0`) becomes the interface. One entry
  per socket, the first process holding it.

No get_kernel_facts on purpose: KernelFactsMixin reports a Linux kernel's
modules and CONFIG_ options, which a BSD kernel does not have.

Fixtures recorded on the FreeBSD 15.1 and OpenBSD 7.9 VMs with test
listeners on 127.0.0.1 and ::1 (NetOrk/netork#798).

README.md

@@ -32,6 +32,18 @@ over SSH, built on [napalm-device-types](https://git.netork.io/NAPALM/napalm-dev
 | `get_arp_table` | ✓ | ✓ | `arp -an` |
 | `get_lldp_neighbors` | `{}` | `{}` | no LLDP daemon in either base system |
 | `get_users`, `get_processes`, `get_cron_jobs` | ✓ | ✓ | `/etc/passwd`+`/etc/group`, `ps … lstart`, system crontab + `crontab -l` |
+| `get_listening_sockets` | ✓ | ✓ | `sockstat -46lq` / `fstat -n` as root, `netstat -an` without |
+
+`get_listening_sockets` has the shape of napalm-device-types'
+`ListeningSocketsMixin` (whose `ss`/cgroup reading is Linux's) and its rule:
+read as root first, without root when that brings nothing back
+(`attributed: False`). FreeBSD's `sockstat` sees every socket either way;
+OpenBSD's `fstat` shows a user only their own processes, so without root the
+sockets come from `netstat -an`, unnamed.
+
+There is deliberately no `get_kernel_facts`: `KernelFactsMixin` reports a
+Linux kernel's modules and `CONFIG_*` options, which a BSD kernel does not
+have, and `hasattr(driver, "get_kernel_facts")` has to stay truthful.
 
 Packages, services, updates (#799) and SNMP (#800) follow.