SSH daemon: change default key size for RSA, add alert for weak keys.
Manual: Fix some links
sshd: Support multiple host keys

The option services.openssh.hostKeys now allows specifying multiple
host keys.  The default value enables both a DSA and ECDSA key.
(Clients by default will use the ECDSA key, unless known_hosts already
has a DSA key for that host.)  To use only an ECDSA key, you can say:

  services.openssh.hostKeys =
    [ { path = "/etc/ssh/ssh_host_ecdsa_key";
        type = "ecdsa";
        bits = 521;
      }
    ];
Fixing handling of parameters with spaces in torsocks/torify
convert bbswitch job to systemd unit

dramatically speeds up my boot time because it was the last
service (for me) that depended on udev-settle.service

udev-settle isn't needed for modern system initialization but some
oldschool services (mdadm/lvm/cryptsetup) depend on it so they can
just enumerate devices instead of having to react to changes
dynamically. In NixOS these things are usually already taken care of
during stage 1 (early ramdisk) if you use them.
power management: provide a post-resume target and unify
suspend/hibernate/hybrid-sleep handling
Add programs.ssh.extraConfig option
Don't try to guess the location of the NixOS config file

The NixOS config need not be $NIXOS_CONFIG, it can also be set through
-I nixos-config=... or not exist in a separate file at all (e.g. in a
NixOps deployment).

Issue #212.
Fix spelling

Also, it's not necessary to order a unit after "sysinit.target" since
that's implied.
Build 32-bit VirtualBox image

Issue #200.
Add vdi.i686-linux to the release-critical jobs
Add some more missing uids/gids
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).