[New Feature] Flush AI units

Description:

AI can hoard quite a bunch of units from finished or aborted missions. It would be really nice if AI can clear/flush these units. This action would be similar to when an AI player has sold its buildings and sends all its units to attack. This can also help AI clear up its base.

Possible Implementation:

[GlobalControls] ; rulesmd.ini
AIFlush=yes      ; enable AI unit flushing, defaults to 'no'
FlushCount=50 ; when AI has 50 unused units it will send them all to hunt in one big wave
FlushTimer=10 ; a flush can occur only after this many minutes have passed

The flush conditions should be checked only when the timer expires to avoid lag.