dstripe: support use for migrating off a given Lustre MDT or OST

[ofaaland]

Either in conjunction with lfs find (see earlier issue) or instead of it, enable migration off a given Lustre MDT or OST.

1. Identify files residing on the given target

• this could be accomplished by accepting a file list from "dfind" if dfind had options for identifying files in this manner

2. restripe via "lfs migrate" equivalent code path instead of "lfs setstripe" and data-copy

• this would enable migrations with significantly less user coordination