Fix exit event handling in shim

Could issues where when exec processes fail the wait block is not
released.

Second, you could not dump stacks if the reaper loop locks up.

Third, the publisher was not waiting on the correct pid.

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

linux/proc/exec.go

@@ -143,6 +143,7 @@ func (e *execProcess) start(ctx context.Context) (err error) {
 		opts.ConsoleSocket = socket
 	}
 	if err := e.parent.runtime.Exec(ctx, e.parent.id, e.spec, opts); err != nil {
+		close(e.waitBlock)
 		return e.parent.runtimeError(err, "OCI runtime exec failed")
 	}
 	if e.stdio.Stdin != "" {