examples/ufs: Add option to set root of the fs tree.
examples/ufs: Add option to set root of the fs tree.

R=go9p, mirtchovski
CC=go9p
http://codereview.appspot.com/6321043

Committer: Andrey Mirtchovski <f170c58f618c89c5450ad9258257833bd0e0686e@gmail.com>

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