Add DeleteProcess API for removing execs

We need a separate API for handing the exit status and deletion of
Exec'd processes to make sure they are properly cleaned up within the
shim and daemon.

Fixes #973

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

plugin/container.go

@@ -33,6 +33,8 @@ type Task interface {
 	CloseStdin(context.Context, uint32) error
 	// Checkpoint checkpoints a container to an image with live system data
 	Checkpoint(context.Context, CheckpointOpts) error
+	// DeleteProcess deletes a specific exec process via the pid
+	DeleteProcess(context.Context, uint32) (*Exit, error)
 }
 
 type CheckpointOpts struct {