Mount shared alloc dir, modified API and tests

client/allocdir/alloc_dir_linux.go

@@ -0,0 +1,18 @@
+package allocdir
+
+import (
+	"os"
+	"path/filepath"
+	"syscall"
+)
+
+// Bind mounts the shared directory into the task directory. Must be root to
+// run.
+func (d *AllocDir) mountSharedDir(taskDir string) error {
+	taskLoc := filepath.Join(taskDir, SharedAllocName)
+	if err := os.Mkdir(taskLoc, 0777); err != nil {
+		return err
+	}
+
+	return syscall.Mount(d.SharedDir, taskLoc, "", syscall.MS_BIND, "")
+}