Witness patch

[Don Lewis]

On  1 Sep, John Baldwin wrote:
> This patch forces witness to complain if any mutex is held when Giant is 
> locked to enforce Giant being the first mutex in the lock order.  This might 
> help track down some of the network LORs being reported recently.
> 
> http://www.FreeBSD.org/~jhb/patches/witness.patch

I think it would make sense to print different messages for the
different trigger conditions.