core.packet: extract packet.account_free from packet.free
core.packet: extract packet.account_free from packet.free

Extract a function packet.account_free from packet.free to expose the
accounting aspects of freeing packets (engine.{frees,freebytes,freebits)) to
apps that might need it (e.g., an app that transfers packets across process
boundaries.)

This is needed because the engine paces its breathing (or rather, estimates
load) based on the counter engine.frees.

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