Update the libufs cgget() and cgput() interfaces to have a similar

Update the libufs cgget() and cgput() interfaces to have a similar
API to the sbget() and sbput() interfaces. Specifically they take
a file descriptor pointer rather than the struct uufsd *disk pointer
used by the libufs cgread() and cgwrite() interfaces. Update fsck_ffs
to use these revised interfaces.

No functional changes intended.

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