DirTree: use [bftrunc] to zero out files/subdirs on creation
DirTree: use [bftrunc] to zero out files/subdirs on creation

makes the proofs work out (using the admitted proof for [bftrunc]).
but this does not yet re-construct the whole-tree invariant; need
loop postconditions for that, as Adam suggested.

mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl