Bring in DIRFS: A filesystem for VKERNELS

* What is DIRFS?
  dirfs is a "filesystem" specific for vkernel(7) which allows mounting
  host's directories into the vkernel. It runs directly in the vkernel's VFS
  code, as any other regular filesystem, but it does syscalls (vkernels are
  userland programs) to retrieve the information needed on every operation
  requested.

  Needless to say that the operations that you can perform in the host
  directories/files depend on the permissions the user that runs the vkernel.
  For example, you will not be able to 'chflags schg' if you run the vkernel
  with a regular user instead of root.

* How does it work?
  It basically works like any other filesystem with the difference that you
  specify the host directory instead of the disk device node. It has its own
  mount_dirfs command that will be called by the system's mount(8) command when
  necessary.
  
	vkernel64 # mount -t dirfs /usr/src2 /mnt
	vkernel64 # df -h /mnt
	Filesystem        Size   Used  Avail Capacity  Mounted on
	dirfs@/usr/src2    47G    36G    12G    75%    /mnt

  Umounting is a normal operation too:

  vkernel64 # mount | fgrep dirfs
	dirfs@/usr/src2 on /mnt (dirfs)
	vkernel64 # umount /mnt

* What's the current status?
  Currently it is in a experimental status, with (probably) many bugs and some parts
  missing.

* Missing stuff
  - It is not possible to make hardlinks on it yet (VOP_NLINK mising, returns EOPNOTSUPP).
  - dirfs root for vkernels is not *yet* supported although I expect it to be.
  - There is no locking on file descriptors between host <-> vkernel.