Import: don't fail for cross-device imports

rename(2) only works on a single device. When trying to import a file
(with move=true) from a different device to the one on which the diskv
is located, we should gracefully fall back to copying the file and then
removing it (like mv(1) does).
Import: do not bust cache unless successful
Merge pull request #19 from jonboulle/rename

Import: don't fail for cross-device imports