Begin adding quota cursor/iteration interface to VFS_QUOTACTL.

Add struct quotakcursor.
Add QUOTACTL_CURSOROPEN and QUOTACTL_CURSORCLOSE operations.
Implement the plumbing for them.
Add trivial implementations of them for quota2.
(iteration is not supported on quota1 for the time being, just as
getall isn't)
Have the proplib interpreter open and close a cursor around doing
QUOTACTL_GETALL.

Note: this change requires a kernel version bump.

(dholland)