/etc/profile: try all nix profiles for ASPELL_CONF

Aspell can only handle one dict-dir directive and currently we hardocde
that to
  ASPELL_CONF="dict-dir $HOME/.nix-profile/lib/aspell"

This means that aspell doesn't work if it is installed to the system or
default nix profile -- it only works in the user profile.

With this change, aspell can be installed to any of the nix profiles. If
it is installed in more than one profile, the most "local" profile wins
(i.e. sysadmin can set up a default, users can override it).
release.nix: Automatically include all of tests/default.nix
Test whether the transparent fetchurl cache (tarballs.nixos.org) works

Testing this is useful in any case, but it's necessary now because Nix
1.6 doesn't check the binary cache for fetchurl output anymore.

http://hydra.nixos.org/build/6144188
nixos-install: Fix copying from the CD

Nix 1.6 doesn't run the copy-from-other-stores substituter by default
anymore, so turn it on explicitly.

http://hydra.nixos.org/build/6144173
Add kexec support

You can now do a fast reboot (bypassing the BIOS, which may take
several minutes on servers) by running ‘systemctl kexec’.

Unfortunately the QEMU test for this is unreliable due to a QEMU bug
(it randomly crashes with a message like ‘Guest moved used index from
8 to 0’), so it's commented out.
Set CURL_CA_BUNDLE env variable for nix-daemon to allow pulling from a binary cache on https. Did not add to nix.envVars to avoid being added to shellInit.
bind: allow forwarders to differ from nameservers
modules/misc/ids.nix: document the fact that the uid for tcpcryptd is hard-coded in the daemon
Regression test for blkio functionality
manual: fix typo, #254.
Merge pull request #257 from oxij/bind-forwarders

bind: allow forwarders to differ from nameservers
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
Generate /etc/passwd and /etc/group at build time

This is a rather large commit that switches user/group creation from using
useradd/groupadd on activation to just generating the contents of /etc/passwd
and /etc/group, and then on activation merging the generated files with the
files that exist in the system. This makes the user activation process much
cleaner, in my opinion.

The users.extraUsers.<user>.uid and users.extraGroups.<group>.gid must all be
properly defined (if <user>.createUser is true, which it is by default). My
pull request adds a lot of uids/gids to config.ids to solve this problem for
existing nixos services, but there might be configurations that break because
this change. However, this will be discovered during the build.

Option changes introduced by this commit:

* Remove the options <user>.isSystemUser and <user>.isAlias since
they don't make sense when generating /etc/passwd statically.

* Add <group>.members as a complement to <user>.extraGroups.

* Add <user>.passwordFile for setting a user's password from an encrypted
(shadow-style) file.

* Add users.mutableUsers which is true by default. This means you can keep
managing your users as previously, by using useradd/groupadd manually. This is
accomplished by merging the generated passwd/group file with the existing files
in /etc on system activation. The merging of the files is simplistic. It just
looks at the user/group names. If a user/group exists both on the system and
in the generated files, the system entry will be kept un-changed and the
generated entries will be ignored. The merging itself is performed with the
help of vipw/vigr to properly lock the account files during edit.
If mutableUsers is set to false, the generated passwd and group files will not
be merged with the system files on activation. Instead they will simply replace
the system files, and overwrite any changes done on the running system. The
same logic holds for user password, if the <user>.password or
<user>.passwordFile options are used. If mutableUsers is false, password will
simply be replaced on activation. If true, the initial user passwords will be
set according to the configuration, but existing passwords will not be touched.

I have tested this on a couple of different systems and it seems to work fine
so far. If you think this is a good idea, please test it. This way of adding
local users has been discussed in issue #103 (and this commit solves that
issue).