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>

task.go

@@ -53,7 +53,7 @@ type Task interface {
 type Process interface {
 	Pid() uint32
 	Start(context.Context) error
-	Delete() error
+	Delete(context.Context) (uint32, error)
 	Kill(context.Context, syscall.Signal) error
 	Wait(context.Context) (uint32, error)
 	CloseStdin(context.Context) error