Add cgroup manipulation functions to libutil.
Add daemon-use-cgroups option into Settings in libstore.
Read croups from calling process of nix-daemon (if enabled by config).
Package nar (xz) archives with -6e instead of -9.
cgroup: Move cgroup handling to separate files in libutil.
cgroup: Add documentation for daemon-use-cgroups option.
libexpr: Fix build (missing semicolon).
Fix assertion failure in ‘nix-store --load-db’

Namely:

  nix-store: derivations.cc:242: nix::Hash nix::hashDerivationModulo(nix::StoreAPI&, nix::Derivation): Assertion `store.isValidPath(i->first)' failed.

This happened because of the derivation output correctness check being
applied before the references of a derivation are valid.