systemd: Fail if kernel features are missing.

This has rendered my system unbootable, because I forgot to enable AUTOFS4 in my
custom kernel. In addition to AUTOFS4, this includes (hopefully) all other
kernel features needed by systemd, as listed in the README:

REQUIREMENTS:
  Linux kernel >= 2.6.39
    with devtmpfs
    with cgroups (but it's OK to disable all controllers)
    optional but strongly recommended: autofs4, ipv6

Autofs4 is not a requirement here, but in our case it turns out that the system
is not able to boot properly with a LUKS-enabled system (or at least not on _my_
system).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
firewall.nix: Don't fail if IPv6 is disabled
Remove unnecessary toPath that breaks with recent nixUnstable
modules/programs/bash/bash.nix: cosmetic indention fix
Add option "environment.binsh" to configure the shell executable used to create the global /bin/sh symlink.
test-instrumentation.nix: Don't start agetty on hvc0
installer test: Don't wait for getty@tty2 because it's started lazily
Update channel URLs
dnsmasq: Add extraConfig option
Make the installer work on systemd

Systemd mounts the root filesystem as a shared subtree, which breaks
recursive bind mounts.
Make install-grub.pl work when $PATH is empty
Use new-style fileSystems
Systemd requires the latest Nix
Remove unnecessary (AFAICT) call to toPath
Installer test: fix nix-push call

http://hydra.nixos.org/build/3331147
Add a GRUB 1 dependency

http://hydra.nixos.org/build/3331139
Merge master.
Fixed the documentation for programs.ssh.forwardX11 to account for the X11 SECURITY extension.
Merge pull request #40 from falsifian/master

Change the default value of programs.ssh.forwardX11 to false.
Add option for specifying shell aliases, environment.shellAliases.