git annex get from exports

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.

Remote/Directory.hs

@@ -44,7 +44,7 @@ gen :: Git.Repo -> UUID -> RemoteConfig -> RemoteGitConfig -> Annex (Maybe Remot
 gen r u c gc = do
 	cst <- remoteCost gc cheapRemoteCost
 	let chunkconfig = getChunkConfig c
-	return $ Just $ exportableRemote $ specialRemote c
+	exportableRemote $ specialRemote c
 		(prepareStore dir chunkconfig)
 		(retrieve dir chunkconfig)
 		(simplyPrepare $ remove dir)