Move Mount into mount pkg

This moves both the Mount type and mountinfo into a single mount
package.

This also opens up the root of the repo to hold the containerd client
implementation.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

mount/mount_unix.go

@@ -0,0 +1,17 @@
+// +build darwin freebsd
+
+package mount
+
+import "github.com/pkg/errors"
+
+var (
+	ErrNotImplementOnUnix = errors.New("not implemented under unix")
+)
+
+func (m *Mount) Mount(target string) error {
+	return ErrNotImplementOnUnix
+}
+
+func Unmount(mount string, flags int) error {
+	return ErrNotImplementOnUnix
+}