networking.interfaces: Provide ifconfig(8) flags, rdomain, description and groups

[klemensn]

OpenBSD, FreeBSD and macOS all show interface flags the same way.
OpenBSD and FreeBSD support descriptions and groups.
OpenBSD has https://man.openbsd.org/rdomain.4 as well.

If these properties are not set or ifconfig does not print them,
they will simply not appear.

This allows OpenVox to act upon such details without shelling out to
custom scripts; without either of this there is currently no way to
ensure (partial) interface state.

Example from OpenBSD:

pair2: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> rdomain 46 mtu 1500
	lladdr fe:e1:ba:d1:c3:a7
	description: gelatod CLAT 464XLAT
	index 6 priority 0 llprio 3
	patch: pair1
	groups: pair
	media: Ethernet autoselect
	status: active
	inet 192.0.0.1 netmask 0xfffffff8 broadcast 192.0.0.7

Result:

 {
   bindings => [
     {
       address => "192.0.0.1",
       netmask => "255.255.255.248",
       network => "192.0.0.0"
     }
   ],
+  description => "gelatod CLAT 464XLAT",
+  flags => [
+    "UP",
+    "BROADCAST",
+    "RUNNING",
+    "SIMPLEX",
+    "MULTICAST"
+  ],
+  groups => [
+    "pair"
+  ],
   ip => "192.0.0.1",
   mac => "fe:e1:ba:d1:c3:a7",
   mtu => 1500,
   netmask => "255.255.255.248",
   network => "192.0.0.0",
+  rdomain => 46
 }

[klemensn]

@buzzdeee FYI

[corporate-gadfly]

TY @klemensn .