services.cgminer.enable: improve description

So that a search for "bitcoin" in the configuration.nix(5) man page
yields a hit.
Write a reboot record to /var/log/wtmp
Remove unused runlevel targets
Log console logins in /var/log/wtmp
Fix service description
Fix broken systemd shutdown services

The services systemd-random-seed-save.service and
systemd-update-utmp-shutdown.service from systemd 203 don't work,
because they have a RequiresMountFor dependency on /var/lib and
/var/log.  Such a dependency produces a cycle, causing systemd to drop
those services:

  Fixing conflicting jobs by deleting job poweroff.target/stop
  Deleting job systemd-poweroff.service/stop as dependency of job poweroff.target/stop
  Deleting job umount.target/stop as dependency of job systemd-poweroff.service/stop
  Deleting job -.mount/start as dependency of job umount.target/stop
  Deleting job systemd-random-seed-save.service/start as dependency of job -.mount/start
  Deleting job systemd-update-utmp-shutdown.service/start as dependency of job -.mount/start

Dropping the RequiresMountFor doesn't work either, because then
/var/... may have meen unmounted or remounted read-only.

Upstream systemd fixes this by doing the actual work from an ExecStop
action in an already-running service than conflicts with
shutdown.target.  So I backported that here.  The main consequence is
that wtmp shutdown records now get written properly.

The main lesson: shutdown.target is useless for pulling in services
during shutdown if they need to write anywhere.
Fix saving the ALSA sound card state

This didn't work reliably because it raced with the remounting of /.
So if you were unlucky, then / was read-only by the time we ran
"alsactl store".  Now the sound card state is saved before anything is
unmounted/remounted.
Fix bug in NetworkManager name server configuration
Radically change the way NixOS handles environment variables and make it possible not to use Bash as the default interactive shell.

This change does two things:

* "NixOSizes" environment variables generation. This allows some more
  error-checking and opens possibilities for a modular environment
  configuration. From now on the most of environment variables are
  generated directly by the nix code. Generating sh code that
  generates environment variables is left in a few places where
  nontrivial access to a local environment state is needed.
* By doing the first change this patch untangles bash from the
  environment configuration and makes it trivial to add a support for
  other non bash-compatible shells.

Now to the sad part. This change is quite large (and I'm not sure it's
possible to split it) and yet is not quite complete, it needs some
changes to nixpkgs to be perfect.
See !!! comments in modules/config/shells-environment.nix.

Main principle behind this change is "change environment generation
and nothing else". In particular, shell configuration principles stay
exactly the same as before.
Add support for Zsh as an alternative (or not) default (or not) interactive shell.
Merge pull request #256 from oxij/shells-environment

I tested the previous "version" and found my environment to be exactly the same.

Let's start discussing possible extensions/improvements somewhere else. For now it's a nice improvement.
In environment.variables, unify "value" and "list"
Add option systemd.automounts, for definining automount units
Don't generate /etc/environment

/etc/environment, if it exists, should be a list of NAME=VALUE pairs,
not a shell script.  See the pam_env manpage.
Make environment variables overridable
Merge tag 'bluenix/v13.09'