from netbsd:

from netbsd:
Bring in several changes from the ccd:
        * Be a bit better with prototypes
        * Use struct dkdevice in vnd_softc.
        * Prevent the unit from being unconfigured while open.
        * Implement a simple locking mechanism and use it for sanity's
          sake.
Still needs more work; needs to support disklabels and the like.