[Nix-dev] x-updates: master merge

[Michael Raskin]

		Hello.
	It happened that running test-eval-release.sh shows some errors
but plain instantiation of eval-release.nix doesn't. Maybe --strict is
making it evaluate things that are passed through on all platforms, but
only evaluated where relevant.
	I decided that this is good enough and pushed the merge. If 
anyone wants to re-merge, feel free to revert, but I hope most of the
changes I made are OK and there are just more fixes that can be done
(maybe not worth doing). I hope that everything builds fine on Hydra,
eventually.
	Do we want "--strict --xml --eval-only" to succeed for non-Linux
platforms? Or is just instantiation enough? In the latter case, I'd add
the option to test-eval-release.sh.