in df(1) when we have multiple filesystem types, and the complications of
handling UFS2 pushes this over the edge.
Use the .../mount/extern.h to get prototypes of the functions we
borrow from there. Constify things to match. (why aren't these
functions in a lib anyway ?)
Make everything static and set WARNS?=5.
The way the "df diskdevice" thing works for unmounted diskdevices
is not very general.
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