Do verification of checksums of annex objects downloaded from remotes.

* When annex objects are received into git repositories, their checksums are
  verified then too.
* To get the old, faster, behavior of not verifying checksums, set
  annex.verify=false, or remote.<name>.annex-verify=false.
* setkey, rekey: These commands also now verify that the provided file
  matches the key, unless annex.verify=false.
* reinject: Already verified content; this can now be disabled by
  setting annex.verify=false.

recvkey and reinject already did verification, so removed now duplicate
code from them. fsck still does its own verification, which is ok since it
does not use getViaTmp, so verification doesn't happen twice when using fsck
--from.

Command/SetKey.hs

@@ -35,7 +35,7 @@ perform file key = do
 	-- the file might be on a different filesystem, so moveFile is used
 	-- rather than simply calling moveAnnex; disk space is also
 	-- checked this way.
-	ok <- getViaTmp key $ \dest ->
+	ok <- getViaTmp DefaultVerify key $ \dest ->
 		if dest /= file
 			then liftIO $ catchBoolIO $ do
 				moveFile file dest