switch-to-configuration: Stop some target units to ensure proper dependency ordering

This is currently only done for network-interfaces.target, but
it should propably be done for most targets.
Add section on network configuration topics
Add an overview of systemctl/loginctl/journalctl
Add a command ‘nixos-help’ that opens the NixOS manual in a browser
Include libsystemd-daemon.so.* in the initrd since dmsetup needs it

Strangely, this is only case after updating systemd to 197, I didn't
change lvm2...
Don't special-case systemd-journald.service and systemd-user-sessions.service
Use the upstream (but patched) sysinit.target
Install {rescue,emergency}.{target,service}

Also, symlink kbrequest.target to rescue.target as suggested by the
systemd.special manpage.  This way, you can start a sulogin rescue
shell by pressing Alt+Up.
Remove handling of "debug2" and "S|s|single" kernel command line options

The "S|s|single" option is handled by systemd (starting
rescue.target).  And the rescue target basically removes the need for
a special debug shell.  (Also, there is "systemd.crash_shell=1" for
starting a shell if systemd crashes.)
Add dd-agent module
Update the troubleshooting section for systemd
xserver: Allow to add extra options to xorg config.

This is especially useful if you want to supply a default XRandR configuration,
where you need multiple "Monitor" sections in order to set properties for
specific CRTCs (if not running in zaphod mode).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
modules/system/boot/loader/grub/memtest.nix: use 'memtest86plus' instead of 'memtest86'

The 'memtest86' package didn't work on any of my machines. 'memtest86plus', on
the other hand, seems to work just fine. Does anyone know why we keep the
seemingly older version around still?
initrd: Don't enable the root shell by default

Starting an authenticated root shell is a security hole, so don't do
it by default.  The kernel command line parameter
‘initrd.shell_on_fail’ restores the original.  (Of course, this only
improves security if you have a password on GRUB to prevent the kernel
command line from being edited by unauthorized users.)
Give our kernel parameters a common prefix ("boot.*")
Guard against portmap and rpcbind both being enabled
Merge branch 'systemd' of github.com:NixOS/nixos into systemd
xserver: Allow to set XRandR multi head layout.

This is currently only a very simple implementation which just recurses a list
of heads that get chained together to the right of the corresponding previous
item of the list.

If I forgot about something in the already existing configuration options,
please let me know or if this commit is useless or a duplicate, feel free to
revert. But by looking at implementation before this commit, I only see zaphod
and/or quirky xinerama-like configuration options.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Revert "xserver: Allow to add extra options ...

This reverts commit 1e741f1572b6793b861e2f9820015475ce339ae0 as it is
unnecessary according to @edolstra, because services.xserver.config from another
module will be merged into the configuration.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Merge pull request #60 from dysinger/virtualbox-guest-additions-vboxsf-group

VirtualBox Guest Additions: add vboxsf group (for automatic mounting of shared folders)