reorganize and refactor lock code

Added a convenience Utility.LockFile that is not a windows/posix
portability shim, but still manages to cut down on the boilerplate around
locking.

This commit was sponsored by Johan Herland.

Utility/LockFile.hs

@@ -0,0 +1,20 @@
+{- Lock files
+ -
+ - Posix and Windows lock files are extremely different.
+ - This module does *not* attempt to be a portability shim, it just exposes
+ - the native locking of the OS.
+ -
+ - Copyright 2014 Joey Hess <joey@kitenet.net>
+ -
+ - License: BSD-2-clause
+ -}
+
+{-# LANGUAGE CPP #-}
+
+module Utility.LockFile (module X) where
+
+#ifndef mingw32_HOST_OS
+import Utility.LockFile.Posix as X
+#else
+import Utility.LockFile.Windows as X
+#endif