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.

Types/Remote.hs

@@ -159,7 +159,7 @@ unVerified a = do
 -- The FilePath will be relative, and may contain unix-style path
 -- separators.
 newtype ExportLocation = ExportLocation FilePath
-	deriving (Eq)
+	deriving (Show, Eq)
 
 data ExportActions a = ExportActions
 	{ exportSupported :: a Bool

Types/TrustLevel.hs

@@ -21,7 +21,7 @@ import Types.UUID
 -- This order may seem backwards, but we generally want to list dead
 -- remotes last and trusted ones first.
 data TrustLevel = Trusted | SemiTrusted | UnTrusted | DeadTrusted
-	deriving (Eq, Enum, Ord, Bounded)
+	deriving (Eq, Enum, Ord, Bounded, Show)
 
 instance Default TrustLevel  where
 	def = SemiTrusted