mod-free-will now measures what virtual players cost

The module counts the CPU time spent on virtual players and the memory added since world startup; the "AI status" panel shows these values.

Per-thread CPU time (CLOCK_THREAD_CPUTIME_ID) summed over: the module update (brains, client-less sessions), intake of packets addressed to them, and each virtual player's Player::Update, bracketed by the core's OnPlayerBeforeUpdate / OnPlayerAfterUpdate hooks.
Nested measurements (a packet sent during an update) are counted once.
Memory: worldserver resident memory minus the value recorded at world startup, before virtual players log in.
New resources block in the module status (existing fields unchanged); ai-status.py averages it over the last minute.
First reading, 98 players: 15.7 % of one core (1.6 % of the machine), 1.3 GB of memory (4 %).