logstash service: make log verbosity configurable.
chicken: Implement support for bootstrap-builds

This is necessary in order to apply custom patches, as chicken needs a
bootstrap build to regenerate *.c files. This is also necessary when
building from git.
Infrastructure to build chicken eggs.
Implement patches.nix to override parts of a derivation
rename patches -> overrides
Add chicken to PATH of wrapped chicken binaries.
New package: egg2nix.

Generates nix-expressions from Chicken Scheme Eggs.
nixos-iso: Add zfs / btrfs to supportedFilesystems
eggDerivation: Move postInstall into installPhase.
Pass chickenEggs as a parameter to egg2nix.

Those eggs are specific to egg2nix - it's unwise to make them public.
Wrap chicken so it can find gcc.
egg2nix: Update to 0.2.

Also regenerate chicken-eggs.nix - the changeset is quite big, but
subsequent changes will be smaller as egg2nix now sorts its output.
Upgrade Pure to 0.63
smpeg: fix smpeg-config
SDL_mixer: add mp3 support
kde installer: Add gparted, nixos manual and konsole in the desktop
Atom: updated to 0.139.0
Update Hawkthorne game to v0.12.0
Merge pull request #4635 from asppsa/pure-0.63

Upgrade Pure to 0.63
Update OpenLDAP from 2.4.39 to 2.4.40