ID computer: Manage positions for latejoiners

First time I used git, probably did something horribly wrong

Details in first post @  http://forums.nanotrasen.com/viewtopic.php?f=16&t=12490&p=195140#p195140

I have disabled the ability to close original positions since you were worried about "lelassistants" griefing. You can now only close positions that you have opened before. The maximum amount of open positions scales with the station's population, so you won't see 50 clown positions