examinekey: Added --migrate-to-backend

examinekey: Added --migrate-to-backend

Note that, the way the SeekInput parser is written to support batch mode,
it's actually possible to do git-annex examinekey
"SHA1--foo foo.tar.gz" --migrate-to-backend=SHA1E

While that might be kind of useful to support multiple migrations not using
batch mode, I have not documented it. It would be better to take pairs of
key and file in that case.