`On Dialog Start` (used in SCPUI and within `popup_init`) reads popup options by affirmative or negative flags, and the popup for exiting the Mainhall did not set these flags. Notably, most popups that use `POPUP_NO` or `POPUP_YES` already, so this PR sets those flags for this popup. Now `On Dialog Start` within `popup_init` has access to the correct flags.
PR tested and works as expected, both in retail UI and SCPUI