feat(network): add *BSD support via an ifconfig fallback

[eric-eisenhart]

We've been running a variant of this code (copied out of netbox-agent several years ago) locally in order to get stuff working on FreeBSD, and I figured would be potentially useful to others and we should submit it upstream.

I used Claude to pull what we'd done on to the latest branch and otherwise clean it up to be good for a PR, but the original core code here was written by me and has been in-use for years on a variety of FreeBSD physical and virtual servers.

Network.scan() read interface facts exclusively from Linux sysfs (/sys/class/net), so it could not run on *BSD. Abstract the OS-specific bits behind small helpers that use sysfs when present and fall back to parsing ifconfig -a otherwise:

• Add netbox_agent/ifconfig.py: parse ifconfig -a for per-interface MAC and MTU (the facts scan() reads from /sys on Linux).
• scan() enumerates interfaces and reads MAC/MTU/bonding/virtual via _use_sysfs()-guarded helpers; the Linux path is behaviorally unchanged.
• ethtool is already skipped when the binary is absent, so *BSD NICs get ethtool=None (interface type falls back to Other), which downstream already handles.
• Guard the MTU update so a missing (None) MTU never clears a known one.
• Add a fixture + test parsing real FreeBSD ifconfig output.