Straightforward enough, except for the needed belt-and-suspenders sanity
checks to avoid foot shooting due to exports not being key/value stores.
* Even when annex.verify=false, always verify from exports.
* Only get files from exports that use a backend that supports
checksum verification.
* Never trust exports, even if the user says to, because then
`git annex drop` would drop content if the export seemed to contain
a copy.
This commit was supported by the NSF-funded DataLad project.