Virtualize lagg(4) cloner. This change fixes a panic when tearing down

Virtualize lagg(4) cloner.  This change fixes a panic when tearing down
if_lagg(4) interfaces which were cloned in a vnet jail.

Sysctl nodes which are dynamically generated for each cloned interface
(net.link.lagg.N.*) have been removed, and use_flowid and flowid_shift
ifconfig(8) parameters have been added instead.  Flags and per-interface
statistics counters are displayed in "ifconfig -v".

CR:	D842