Panel: total CPU cost of virtual players

The CPU percentage shown will be the worldserver's usage minus its idle usage, indirect cost included (zones and creatures kept active, update broadcasting).

The module's direct measurement only saw about 16 % of one core, versus about 90 % for the whole worldserver: most of the virtual players' cost is indirect.
ai-status.py now computes: worldserver CPU − idle usage recorded at startup, before they log in. That reading will be added to the module by the session in charge of it; until then, the direct measurement is shown.
After a worldserver restart, the last value stays on screen until the first average, instead of disappearing.