imln

I know this is super random, but I've ended up using imln quite a lot (I think I may be the only one ;-p), and have ended up with a version that's diverged a fair bit from the FSL version. I thought the new Git system seemed like a good opportunity to see if it's of wider use.

Key changes - in short, the aim is to make it more of a drop-in replacement for ln:

• Properly handles the case where source is relative to target, rather than the current working directory.
• Exit codes now indicate if link creation was successful.