More flexible handling of target ports and arrival points

I've made some modifications that allow ports to have null arrival points, which will cause incoming players to be positioned relative to the target port the same way they departed their outgoing port.

For example, if the departure and arrival ports contain identical models of a bus, the player will be positioned in the same seat they occupied upon departure, looking the same direction relative to the bus. I use this feature to transport players between identical space shuttles on my server's space map.

Additionally, my changes automatically keep port destinations in sync with the arrival points of their target ports, and I fixed a small typo too.