change load order for campaign loading to a full check: Player-specified -> BUILTIN_CAMPAIGN -> First Avaiable.
clean up the getting of a list of available campaigns
fix simroom issue where single missions, with the [V] icon, would display wrong (this was a retail bug, but it doesn't show normally)
fix bug where, if a campaign failed to load, it would still appear available for savefile useage
fix bug where, when resetting the campaign info, the num_missions var wasn't 0'd and it could cause a sexp Assert() during reset