Allow disabling of flushing when large ships would sail _with_ the induced current

[jackvreeken]

When flushing with the doors on e.g. the fresh side open, large ships sailing in would have a hard time steering/mooring. Large ships generally do like opposite flow, as that makes them more stable.

Because this has an effect on how effective flushing is, it would be nice to implement something like:

• (Optional) disabling flushing ingoing ships on fresh side, when volume above *
• (Optional) disabling fo flushing when ships sailing out on salt side.