This led to the container test failing, which made no sense
whatsoever, until I realized nix-daemon.socket creates the socket
directory as a side effect, which systemd-nspawn then bind-mounts.
http://hydra.nixos.org/build/10397575
If you define a unit, and either systemd or a package in
systemd.packages already provides that unit, then we now generate a
file /etc/systemd/system/<unit>.d/overrides.conf. This makes it
possible to use upstream units, while allowing them to be customised
from the NixOS configuration. For instance, the module nix-daemon.nix
now uses the units provided by the Nix package. And all unit
definitions that duplicated upstream systemd units are finally gone.
This makes the baseUnit option unnecessary, so I've removed it.
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
Suggested by Marc Weber. Fixes #1059.
Generate /etc/nix.machines only if buildMachines is not empty. Thus,
if you want to manage /etc/nix.machines in some other way, you can set
nix.distributedBuilds to true but not set nix.buildMachines.
