add GETAVAILABILITY to external special remote protocol

And some reworking of types, and added an annex-availability git config
setting.

Types/Availability.hs

@@ -0,0 +1,11 @@
+{- git-annex remote availability
+ -
+ - Copyright 2014 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Types.Availability where
+
+data Availability = GloballyAvailable | LocallyAvailable
+	deriving (Eq, Show, Read)