Commits 79117d56, a2dd14f6 and d665a9f0 add the ability to enable/disable an
event trigger (from C code and a Lua code), and save its state in the savegame.

A new 'Single activation=1' attribute can be added to a trigger definition (in
events.dat). When that attribute is set, the trigger is automatically disabled
after its first activation.

HFF traps can now be rewritten to use that attribute, to fix and close this ticket.