Support hot-swapping of removable drives containing gcrypt repositories.

To support this, a core.gcrypt-id is stored by git-annex inside the git
config of a local gcrypt repository, when setting it up.

That is compared with the remote's cached gcrypt-id. When different, a
drive has been changed. git-annex then looks up the remote config for
the uuid mapped from the core.gcrypt-id, and tweaks the configuration
appropriately. When there is no known config for the uuid, it will refuse to
use the remote.

Remote/WebDAV.hs

@@ -46,10 +46,10 @@ remote = RemoteType {
 	setup = webdavSetup
 }
 
-gen :: Git.Repo -> UUID -> RemoteConfig -> RemoteGitConfig -> Annex Remote
+gen :: Git.Repo -> UUID -> RemoteConfig -> RemoteGitConfig -> Annex (Maybe Remote)
 gen r u c gc = new <$> remoteCost gc expensiveRemoteCost
   where
-	new cst = encryptableRemote c
+	new cst = Just $ encryptableRemote c
 		(storeEncrypted this)
 		(retrieveEncrypted this)
 		this