Add service.logind.extraConfig option

So that we can customize systemd-logind in configuration.nix.

Example:
  services.logind.extraConfig = "HandleLidSwitch=ignore";

See man logind.conf for available options.
bring bridge interfaces up

The bridge is not becoming operational until it is brought into the UP state.
When the bridge is statically configured, this is mandatory.
modules/testing/minimal-kernel: linuxPackagesFor no longer takes a 'self' argument
systemd-logind: fix config file header: [Logind] => [Login]

man logind.conf clearly states that the header is [Login] (no 'd').
Without this fix services.logind.extraConfig does not take effect
because logind ignores the invalidly named section.
Add a module for setting sysctl parameters

This adds an option ‘boot.kernel.sysctl’ and generates a file
/etc/sysctl/nixos.conf read by systemd-sysctl.service.
Enable hardlink and symlink restrictions

This prevents many time-of-check-time-of-use security bugs.  Ubuntu
enables these by default as well so they shouldn't cause many
problems.
installation-cd-base.nix: Use boot.kernel.sysctl
add most basic nginx service
Merge pull request #114 from iElectric/nginx-service

Add most basic nginx service
Add lighttpd web server module
Merge pull request #115 from bjornfor/lighttpd-service

Add lighttpd web server module
xfce: Set GTK_PATH to system.path/lib/gtk-2.0, so xfce can find all theme engines.
Add our firmware path to the in-kernel lookup path

Newer kernels (since torvalds/linux@abb139e75c2cdbb955e840d6331cb5863e409d0e) try to
read firmware directly from the filesystem before falling back to a
userspace helper (udev) if firmware cannot be found (in even newer
kernels, the fallback path can be disabled altogether). By default, only
certain paths in /lib/firmware* are searched, so this was initially not
helpful for NixOS.

Since torvalds/linux@27602842060484b564cd725241b402b0bddfb830 (which,
based on the commit message, was implemented just for NixOS, go us!),
though, an extra path can be dynamically prepended to the search path.
So do that, in three ways:

1. Pass a kernel command line option in case the module is built-in
2. Add a line to modprobe.conf in case the module isn't yet loaded by
activation-time
3. Add an activation script to set the option in /sys in case the module
is already loaded by activation-time.
systemd-sysctl.service: Don't depend on basic.target