bsd: Pass '-W' to netstat for full interface names on FreeBSD

By default, `netstat` on FreeBSD only shows the first five characters of an interfaced name, and truncates the rest.  So following the standard VLAN naming convention (`<iface>.<vlan_id>`), we have interface names like `re1.200`, `re1.210`, and `re1.220`.  These are seen as a single interface called `re1.2` by the agent.

Unfortunately, at least OpenBSD has a differing interpretation of `-W` for netstat, so limit the additional flag to just FreeBSD for now.  I don't have access to any other BSD to test right now.  I'm also not sure this is the best way to handle this, but it seemed reasonably clean.
Just to make the code more readable, we could modify a `netstat_flags` in the if block and only call once `subprocess` after that. What do you think?