sbin/newfs_hammer: Explicitly tell which volume is root volume

sbin/newfs_hammer: Explicitly tell which volume is root volume

hammer volume-del says "Cannot remove root-volume" if one
tries to remove the root volume, however no hammer commands
actually tell which one is the root volume.

This commit makes newfs_hammer printf the root volume name,
followed by the existing undo/etc info of the root volume.
Also refer to the root volume in manpages.

(It may be helpful if other hammer commands like info or
volume-list show the root volume name just like newfs_hammer,
however it's not too trivial to do that. One reason is because
hammer seems to have been designed that way so users don't
need to care about any detail of underlying fs volumes.
This commit is trivial since newfs_hammer code knows which
one is the root volume)