Fixes and improvements based on experience

[ran-lasgalen]

I've used your script to set up my system, and found that some errors may be easily fixed and some improvements make it more usable in different setups. So I suggest them, split in several commits so you may select what you think appropriate.

[hn]

Hi, thank you very much for your contribution and especially for the fine-grained commits.

I've merged most of the commits except:
339d459: I have to think about that. If we use external env variables, I would prefer a more consistant and well-considered naming scheme, e.g. TARGET_PARTBIOS, TARGET_HOSTNAME, ...
5c3111b: I haven't used autoexpand=on so far and I don't know if it's a good idea to enable it by default