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
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).