port transferkeys to windows; make stopping in progress transfers work too (probably)

transferkeys had used special FDs for communication, but that would be
quite annoying to do in Windows.

Instead, use stdin and stdout. But, to avoid commands like rsync stomping
on them and messing up the communications channel, they're duplicated to a
different handle; stdin is replaced with a null handle, and stdout is
replaced with a copy of stderr. This should all work in windows too.

Stopping in progress transfers may work on windows.. if the types unify
anyway. ;) May need some more porting.

GitAnnex.hs

@@ -1,6 +1,6 @@
 {- git-annex main program
  -
- - Copyright 2010 Joey Hess <joey@kitenet.net>
+ - Copyright 2010-2013 Joey Hess <joey@kitenet.net>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -23,9 +23,7 @@ import qualified Command.Get
 import qualified Command.FromKey
 import qualified Command.DropKey
 import qualified Command.TransferKey
-#ifndef mingw32_HOST_OS
 import qualified Command.TransferKeys
-#endif
 import qualified Command.ReKey
 import qualified Command.Reinject
 import qualified Command.Fix
@@ -129,9 +127,7 @@ cmds = concat
 	, Command.FromKey.def
 	, Command.DropKey.def
 	, Command.TransferKey.def
-#ifndef mingw32_HOST_OS
 	, Command.TransferKeys.def
-#endif
 	, Command.ReKey.def
 	, Command.Fix.def
 	, Command.Fsck.def