Changed IEDs into firebombs.

Changed IEDs into firebombs to curtail the amount of station damage done whilst keeping IEDs useful and still keeping fuel tank bombs a useful saboteur's tool.

Their fuse time is a bit more variable too, including a 1 in 10 chance of them going off practically straight away (note: this is counter-able by the user wearing a fire-suit, git gud and l2p)