Add customized separator for accumulate droption

Created by: Louis-Ye

The current droption_t uses space as the separator for option value when the DROPTION_FLAG_ACCUMULATE is set. This prevents the accumulate option from having value that has space inside. e.g., --accumulate_op "item1" --accumulate_op "item2.1 item2.2" will assign the value of accumulate_op option to "item1 item2.1 item2.2", which looks like a 3-item value and is not as expected. We need a separator other than space to separate the value.