Fixes and improvements based on experience

[ran-lasgalen]

trafficstars

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: https://github.com/hn/debian-stretch-zfs-root/pull/4/commits/339d45943f12ce3ede1d6a54bd301622a9f3eac9: 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, ... https://github.com/hn/debian-stretch-zfs-root/pull/4/commits/5c3111bd495c96be716efbbe44e9ca4e7f022bf5: I haven't used autoexpand=on so far and I don't know if it's a good idea to enable it by default