V_rootvnode

[James Gritton]

In merging vimage into jails, there's one field prison has that vimage 
lacks: pr_root.  In planning for the merge, it would make sense to treat 
rootvnode like the pr_root of the base prison, i.e. use a V_rootvnode macro.

As with the hostname, there are many places in the kernel where there's 
no reasonable context for anything other than the real root vnode.  So 
in addition to the V_rootvnode, we'd want a G_rootvnode like the 
recently introduced G_hostname.

- Jamie