let Remote.availability return Unavilable

This is groundwork for making special remotes like borg be skipped by
sync when on an offline drive.

Added AVAILABILITY UNAVAILABLE reponse and the UNAVAILABLERESPONSE extension
to the external special remote protocol. The extension is needed because
old git-annex, if it sees that response, will display a warning
message. (It does continue as if the remote is globally available, which
is acceptable, and the warning is only displayed at initremote due to
remote.name.annex-availability caching, but still it seemed best to make
this a protocol extension.)

The remote.name.annex-availability git config is no longer used any
more, and is documented as such. It was only used by external special
remotes to cache the availability, to avoid needing to start the
external process every time. Now that availability is queried as an
Annex action, the external is only started by sync (and the assistant),
when they actually check availability.

Sponsored-by: Nicholas Golder-Manning on Patreon

Remote/External.hs

@@ -68,7 +68,7 @@ gen r u rc gc rs
 	| externaltype == "readonly" = do
 		c <- parsedRemoteConfig remote rc
 		cst <- remoteCost gc c expensiveRemoteCost
-		let rmt = mk c cst GloballyAvailable
+		let rmt = mk c cst (pure GloballyAvailable)
 			Nothing
 			(externalInfo externaltype)
 			Nothing
@@ -87,7 +87,6 @@ gen r u rc gc rs
 			(Git.remoteName r) (Just rs)
 		Annex.addCleanupAction (RemoteCleanup u) $ stopExternal external
 		cst <- getCost external r gc c
-		avail <- getAvailability external r gc
 		exportsupported <- if exportTree c
 			then checkExportSupported' external
 			else return False
@@ -107,7 +106,7 @@ gen r u rc gc rs
 		let cheapexportsupported = if exportsupported
 			then exportIsSupported
 			else exportUnsupported
-		let rmt = mk c cst avail
+		let rmt = mk c cst (getAvailability external)
 			(Just (whereisKeyM external))
 			(getInfoM external)
 			(Just (claimUrlM external))
@@ -777,25 +776,16 @@ getCost external r gc pc =
 		return c
 	defcst = expensiveRemoteCost
 
-{- Caches the availability in the git config to avoid needing to start up an
- - external special remote every time time just to ask it what its
- - availability is.
- -
- - Most remotes do not bother to implement a reply to this request;
+{- Most remotes do not bother to implement a reply to this request;
  - globally available is the default.
  -}
-getAvailability :: External -> Git.Repo -> RemoteGitConfig -> Annex Availability
-getAvailability external r gc = 
-	maybe (catchNonAsync query (const (pure defavail))) return
-		(remoteAnnexAvailability gc)
+getAvailability :: External -> Annex Availability
+getAvailability external = catchNonAsync query (const (pure defavail))
   where
-	query = do
-		avail <- handleRequest external GETAVAILABILITY Nothing $ \req -> case req of
-			AVAILABILITY avail -> result avail
-			UNSUPPORTED_REQUEST -> result defavail
-			_ -> Nothing
-		setRemoteAvailability r avail
-		return avail
+	query = handleRequest external GETAVAILABILITY Nothing $ \req -> case req of
+		AVAILABILITY avail -> result avail
+		UNSUPPORTED_REQUEST -> result defavail
+		_ -> Nothing
 	defavail = GloballyAvailable
 
 claimUrlM :: External -> URLString -> Annex Bool