Enable buying laser for rear mount

Fixes #3064

I've tested it and it seems to work, by that I mean it adds a rear laser if the forward one is occupied. And it fires when using it when in rear view.

Regarding the code: maybe I should add an `EquipType:GetSlots(ship)` and use that instead of accessing it directly `e.slots`?

`pairs` doesn't provide any guarantees about the order of iteration, so this should instead be a numeric for loop (or use `ipairs`)