Label and group interfaces created by pot

[grembo]

When running a large number of pots, it gets hard to easily figure out which
epair interface was created for which pot, especially when debugging and if
the jail is not running anymore.

This change adds two attributes to all interfaces created by pot:

1. A description, which is the pot name in case pots/epair interfaces and
   the bridge's purpose in case of bridges.
2. The group "pot"

With the change, it's easy to find interfaces belonging to pot

$ ifconfig -g pot
bridge0
epair2a
epair3a
epair4a
epair5a
epair6a
epair7a
epair8a
epair9a

and also easier to figure out what their purpose is

$ ifconfig bridge0 | grep -B1 descr
bridge0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0...
    description: pot public bridge

$ ifconfig epair2a
epair2a: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> ...
        description: my-pot_96958fcf_1401fde2-d6e9-f12e-9c19-08baf0195561
        options=8<VLAN_MTU>
        ether 02:5d:2b:d4:4e:0a
        groups: epair pot
        media: Ethernet 10Gbase-T (10Gbase-T <full-duplex>)
        status: active
        nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>

$ ifconfig -g pot | xargs -n1 ifconfig | grep -B1 my-pot
epair2a: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> ...
        description: my-pot_96958fcf_1401fde2-d6e9-f12e-9c19-08baf0195561

The information contained in descriptions/groups could also be taken advantage
of when developing a more robust pot stop procedure.

[pizzamig]

This is a great idea and stop can be way more selective on its implementation.
Also the _epair_cleanup function would benefit from it.