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.