Journaling layer work.

Journaling layer work.

* Adjust the new mountctl syscall to make the passed file descriptor an
  explicit argument rather then storing the fd in the control structure.
  Convert the fd to a file pointer to make kern_mountctl() callable from
  a pure thread.

* Get rid of vop_stdmountctl and just have the VOP default ops call
  journal_mountctl(), which makes things less confusing.

* Get more of the journaling infrastructure working.  Basic installation
  and removal of the journaling structure and the creation and destruction
  of the worker thread and stream file pointer now works (with lots of XXX's).

* Add a journaling vector for VOP_NMKDIR to test the journaling VOP ops shim.