Improve siege order handling while packing

[historic_bruno]

If giving orders while packing/unpacking, it might waste time re-packing afterward:

• If ordered to move while unpacking, it should perhaps cancel unpacking and move
• If ordered to attack in-range targets while packing, it should perhaps cancel packing and attack

This could potentially be very yucky, because it will add even more special cases when adding new orders to determine if they should be queued or preempt the current packing order. I almost think it would be best to improve the order queue behavior at the same time, if possible.