hammer2 - initial mkdir skeleton

hammer2 - initial mkdir skeleton

* Code up VOP_NMKDIR() to test the chain creation code and VOP_READDIR.

* Code up VOP_NLOOKUPDOTDOT.

* Adjust newfs_hammer2 to set alloc_tid to 16, so we can reserve some
  inode numbers for the super-root and root.

* Add another field to the media inode_data for inode allocation within
  a PFS.

Note: These are mostly skeletons.  We don't flush modified chain elements
      back to the disk yet and the chain create path can't create indirect
      blocks yet.