mint-arena: Fix spawn/freed entity logic (specifically harvester skulls)

- Actually use the second 'force' pass in G_Spawn when out of available slots.
- Make G_EntitiesFree return qtrue if we can open a new slot. (Only used when spawning Harvester skulls.)

Fixes not spawning Harvester skulls when there are no 'open freed slots', but we have other slots available to open.