Fix up EV_* text for new events.
Overmind, reactor & repeaters: warn about destroyed buildings within range.

The warnings are rate-limited; for any given area, they won't occur more
than once every 15s.

For aliens, buildings regarded as inside the base will be warned about as
the base being under attack.

For humans, buildings within the reactor's power radius or regarded as
inside the base will be reported as the base being under attack. Buildings
withing repeater power range will be warned about via the nearest repeater;
warnings are limited on a per-area basis (using area names) if possible,
else per-repeater (which could become spammy).

The defence computer is no longer used for ‘base under attack’ warnings.
Implement a fake location entity; use it to avoid ‘under attack’ spam.