Fix: Really check for start_date once an AI company slot becomes available.

[SamuXarick]

basically, if there's no slots, it checks daily, using the interval 3601 - 3600
and never reaches 0
if it gets a start_date lower than that 3601-3600 interval, then the real countdown starts, using that start_date value. When it reaches 0, it tries to make ai company

[LordAro]

This seems like a hack to me. The better solution is to redo the whole start_date thing to be more sane, understandable and predictable - this start_date thing has come up far too often

[stale]

This pull request has been automatically marked as stale because it has not had any activity in the last month.
Please feel free to give a status update now, ping for review, or re-open when it's ready.
It will be closed if no further activity occurs within 7 days.
Thank you for your contributions.

[glx22]

The rework done in #7661 seems better to me, so I'm closing this PR.