Make passing the methods in a cdevsw structure optional.
Move "CANFREE" and "NOGIANT" flags into struct disk instead of the
cdevsw which may or may not be there.
Rename CANFREE to CANDELETE to match BIO_DELETE operation.
Add "OPEN" flag so drivers don't have to provide open/close methods
just to maintain such a flag.
Add temporary stopgap include of <sys/conf.h> to <sys/disk.h> until
the files which have them in the other order are fixed.
Add KASSERTS to make sure we don't get fed too many NULL pointers.
Clear our geom's softc pointer before we wither.