First attempt at using systemd

Basic booting works.  Systemd starts agetty instances on tty1 and
tty2.  Shutdown and journald also work.
Merge branch 'master' of github.com:NixOS/nixos into systemd
Get lots more systemd stuff working

Enabled a bunch of units that ship with systemd.  Also added an option
‘boot.systemd.units’ that can be used to define additional units
(e.g. ‘sshd.service’).
Use udev from systemd
Use kmod instead of module-init-tools
Use pam_systemd.so to set up device ownership

This removes the need for ConsoleKit, so it's gone.
To ease migration to systemd, generate units from the ‘jobs’ option

Also get rid of the ‘buildHook’ job option because it wasn't very useful.
Add a ‘wantedBy’ attribute to unit definitions

This attribute allows a unit to make itself a dependency of another unit.

Also, add an option to set the default target unit.
Improve the dependencies of units generated by the Upstart emulation
Add an option ‘boot.systemd.services’

This option makes it more convenient to define services because it
automates stuff like setting $PATH, having a pre-start script, and so on.
Upstart ‘boot.systemd.services’ for the Upstart compatibility layer
Rename mingetty module to agetty
Whitespace
Get rid of the Upstart shutdown job

The only thing that Upstart's shutdown job did that systemd doesn't do
natively is update the hardware clock.  So added a service for that.
Add a .gitignore
Don't put quotes around environment values
Use socket-based activation of the Nix daemon
Don't run syslogd and klogd

The systemd journal removes the need for running syslogd and klogd, so
don't start them.
Remove accidentally committed line
Set the default unit to "graphical.target" if X11 is enabled