atd: don't enable at daemon by default

The at daemon doesn't work on NixOS [1], so enabling it by default
doesn't seem useful. I'd argue that it shouldn't be enabled by default
even if it worked, actually.

[1] http://lists.science.uu.nl/pipermail/nix-dev/2013-April/011048.html
Adding iw to systemPackages.
lighttpd: add cgit sub-service

(cgit is "a hyperfast web frontend for git repositories written in C")

cgit is enabled like this (assuming lighttpd is already enabled):

  services.lighttpd.cgit.enable = true;

and configured verbatim like this (contents of the cgitrc file):

  services.lighttpd.cgit.configText = ''
    cache-size=1000
    scan-path=/srv/git
  '';

cgit will be available from this URL: http://yourserver/cgit

In lighttpd, I've ensured that the cache dir for cgit is created if cgit
is enabled.
lighttpd: gitweb: add extraConfig option

So that we can append custom configuration text to the end of the
generated gitweb.conf file.
Merge pull request #179 from bjornfor/lighttpd-gitweb-improvements

lighttpd: gitweb: add extraConfig option
Merge pull request #178 from bjornfor/lighttpd-cgit-subservice

lighttpd: add cgit sub-service
Fixed disnix service to use systemd's dependency facilities
Generate /etc/passwd and /etc/group statically
Add users.mutableUsers, to allow using useradd command. WIP, doesn't work at all yet.