Change stdenv on darwin to not set NIX_ENFORCE_PURITY
unbound: update from 1.4.21 to 1.4.22, service from Upstart to systemd
Merge pull request #2788 from jwiegley/stdenv

Change stdenv on darwin to not set NIX_ENFORCE_PURITY.

Further details are at <https://github.com/NixOS/nixpkgs/pull/2788>.
Merge pull request #2790 from ehmry/unbound

unbound: update from 1.4.21 to 1.4.22, service from Upstart to systemd
Build Haddocks with source hyperlinks unless doHscolour is false