PERFORCE change 161987 for review

[Julian Elischer]

Marko Zec wrote:
> http://perforce.freebsd.org/chv.cgi?CH=161987
> 
> Change 161987 by zec at zec_tpx32 on 2009/05/12 18:47:49
> 
> 	Back out O(n**2) ad-hoc hack for searching for available
> 	ifunits in cloning ifnets, and restore the standard O(n)
> 	bitmapped searching / ifunit allocation method for both
> 	default and options VIMAGE builds.
> 	
> 	HOWEVER, hereby we also introduce per-vnet if_clone driver
> 	registration and ifunit allocation.  As a (necessary) example,
> 	if_loop is modified to attach itself as an independent
> 	cloner instance to each vnet.
> 	
> 	This approach has a neat byproduct: if_clone drivers that
> 	do not explicitly declare themselves as multi-vnet, by
> 	exporting an iattach() method and registering to the vnet
> 	framework, continue to work with unmodified semantics in
> 	the default vnet.  However, they will NOT be available
> 	in other vnets.
> 	
> 	This brings us a step closer to being able to selectively
> 	attach subsystems to particular vnets, instead of having
> 	all subsystems unconditionally available to all vnets by
> 	default.
>

excellent sidestep.