Extract volume interaction to a volumes service

This cleans up some of the package API's used for interacting with
volumes, and simplifies management.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>
Upstream-commit: e4b6adc88e967971de634596654d9bc33e7bd7e0
Component: engine

components/engine/pkg/directory/directory_unix.go

@@ -1,4 +1,4 @@
-// +build linux freebsd
+// +build linux freebsd darwin
 
 package directory // import "github.com/docker/docker/pkg/directory"