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.

Remote/Git.hs

@@ -626,10 +626,11 @@ copyToRemote' repo r (State connpool duc _) key file meterupdate
 				ensureInitialized
 				copier <- mkCopier hardlink params
 				let verify = Annex.Content.RemoteVerify r
+				let rsp = RetrievalAllKeysSecure
 				runTransfer (Transfer Download u key) file stdRetry $ \p ->
 					let p' = combineMeterUpdate meterupdate p
 					in Annex.Content.saveState True `after`
-						Annex.Content.getViaTmp verify key
+						Annex.Content.getViaTmp rsp verify key
 							(\dest -> copier object dest p' (liftIO checksuccessio))
 			)
 	copyremotefallback p = Annex.Content.sendAnnex key noop $ \object -> do