HEADSUP: Network interface cloning changes

I just commited network interface cloning.  For most users there are no
effects.  The single real exception is that users who create their
stf(4) interface with the command "ifconfig stf" will get an interface
named stf rather then stf0 and ifconfig will not print "stf0" to stdout.

For the record: THERE ARE NO OTHER CHANGES TO THINGS THAT PREVIOUSLY
WORKED!!!

New features:
 - Creation and configuration of vlans can be accomlished with "ifconfig
   <interface>.<tag> create".  There is no support for this in etc yet
   so it's not fully useable.
 - stf(4) interfaces can be created with "ifconfig 6to4" inaddition to
   "ifconfig stf" and "ifconfig stf0".  This mostly exists as an easier
   to understand example of the new cloning code then that provided by
   the new vlan code.
 - Attempting to "ifconfig lo0 destory" now fails rather then panicing
   the machine.

This change will require a recompile of modules using cloning and
modification of external ones to the new API.  __FreeBSD_version has
been bumped to 502119 to mark this change.

-- Brooks

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