enforce retrievalSecurityPolicy

Leveraged the existing verification code by making it also check the
retrievalSecurityPolicy.

Also, prevented getViaTmp from running the download action at all when the
retrievalSecurityPolicy is going to prevent verifying and so storing it.

Added annex.security.allow-unverified-downloads. A per-remote version
would be nice to have too, but would need more plumbing, so KISS.
(Bill the Cat reference not too over the top I hope. The point is to
make this something the user reads the documentation for before using.)

A few calls to verifyKeyContent and getViaTmp, that don't
involve downloads from remotes, have RetrievalAllKeysSecure hard-coded.
It was also hard-coded for P2P.Annex and Command.RecvKey,
to match the values of the corresponding remotes.

A few things use retrieveKeyFile/retrieveKeyFileCheap without going
through getViaTmp.
* Command.Fsck when downloading content from a remote to verify it.
  That content does not get into the annex, so this is ok.
* Command.AddUrl when using a remote to download an url; this is new
  content being added, so this is ok.

This commit was sponsored by Fernando Jimenez on Patreon.

Command/SetKey.hs

@@ -33,7 +33,7 @@ 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 DefaultVerify key $ \dest -> unVerified $
+	ok <- getViaTmp RetrievalAllKeysSecure DefaultVerify key $ \dest -> unVerified $
 		if dest /= file
 			then liftIO $ catchBoolIO $ do
 				moveFile file dest