zabbix: Apply fix for a SQL injection bug

https://support.zabbix.com/browse/ZBX-7091

CVE-2013-5743
transmission: unify versions to the newest, use gtk3

Also introduce transmission_gtk attribute and signify gtk client support
in the name (so newbies using package names can see there's a gui version).
transmission: fix non-gtk builds

I'm sorry, I was incautious.
add sshuttle: Transparent proxy server that works as a poor man's VPN
Some description fixes

There are many more packages to fix, this is just a start.

Rules:
 * Don't repeat the package name (not always that easy...)
 * Start with capital letter
 * Don't end with full stop
 * Don't start with "The ..." or "A ..."

I've also added descriptions to some packages and rewritten others.
Merge pull request #1044 from bjornfor/description-fixes

Some description fixes
rsnapshot: configurable configuration file path

default value: /etc/rsnapshot.conf
Merge pull request #1045 from aristidb/rsnapshot-config

Let rsnapshot use /etc/rsnapshot.conf as configuration instead of non-existent ${rsnapshot}/etc/rsnapshot.conf
firebird service
many suggestions contributed by bjornfor and edolstra
rsnapshot 1.3.1
Merge pull request #1047 from aristidb/master

rsnapshot 1.3.1
Merge pull request #209 from MarcWeber/submit/firebird

systemd/firebird
Make eval-release.nix less picky

It now only checks that each derivation's drvPath attribute evaluates.
Previously it checked everything (via toXML) but that causes
derivations with dubious passthru attribute to fail (see commit
70fee5da112b8a0093d59ac16cdead4d0a72f357).
Drop --xml, it's no longer needed
polkit, pulseaudio: Drop some Linux-specific dependencies
Remove the nixos.unstable job from the trunk-combined jobset
Properly wipe .compose-cache.
Fixes #267.
Move all of NixOS to nixos/ in preparation of the repository merge
Merge the NixOS repository into Nixpkgs
Move pkgs/lib/ to lib/