perl: Call loadConfFile() in doInit to avoid screwing sqlite journal mode.

If the database is opened through perl bindings (and even though nix.conf has
use-sqlite-wal set to false), the database is automatically converted into WAL
mode. This makes the next nix process to access the database convert it back to
"truncate". If the database is still open at the time in wal mode by the perl
program, this fails and crashes the nix doing the wal -> truncate conversion.
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.