add GPS jamming and effects

[Jagohu]

Please consider adding the possibility of GPS jamming in certain areas. It could be added in the mission editor/LUA script with a trigger zone defined for the jamming and simple triggers of GPS jamming ON, GPS jamming OFF.

On the aircraft side it should affect the relevant systems, ie. GPS position lost/highly inaccurate and generated error messages, along with the inability to deliver GPS-guided munitions. A substantial part of ie. the airspace near Syria is GPS jammed area for years now.