Disable exec symlink resolution for PostgreSQL 9.2

When building PostgreSQL with plugins under NixOS, NixOS will create a
postgresql-and-plugins directory which symlinks PostgreSQL and all the plugins
into a single directory. Unfortunately, the plugins will not actually be usable
by PostgreSQL because it will still try and locate them in the original
PostgreSQL share directory, not postgresql-and-plugins.

In this commit, I have patched resolve_symlinks to always return success, which
matches the behavior if HAVE_READLINK is false (so presumably invalid paths are
never passed to this function).
libyaml-cpp: add versions 0.5.1 and 0.3.0

The two versions are actively maintained, and the API changed between them.
SDL2: add release candidate

The tarball is not marked as such, but current 2.0.0 version is still a release
candidate for SDL2.
Add SDL2_image and SDL2_mixer, release candidate.
Add SDL2_gfx (svn version)

There is no package yet, adding latest revision.
SDL2_gfx: add simlinks for headers with SDL_*

All the SDL2 libraries distribute headers with SDL_* names, under SDL2. Do the
same for the gfx one. I told upstream, but no changes so far.
SDL2: update hashes for new stable release tarballs.

I also had to update the url to the main SDL2 lib.
Make glib_networking available for dwbem
flightgear: 2.10.0 -> 2.12.0 (and make it run again)

The current FlightGear expression doesn't work:

  bash$ nix-build -A flightgear
  /nix/store/3c47sibzb9h1vhnn0gnh692hkpjc5p5v-flightgear-2.10.0

  bash$ ./result/bin/fgfs
  ERROR: The path '/nix/store/3c47sibzb9h1vhnn0gnh692hkpjc5p5v-flightgear-2.10.0/lib/FlightGear' does not exist in the file system.

  Base package check failed:
    Version [none] found at: /nix/store/3c47sibzb9h1vhnn0gnh692hkpjc5p5v-flightgear-2.10.0/lib/FlightGear
    Version 2.10.0 is required.
  Please upgrade/downgrade base package and set the path to your fgdata
  with --fg-root=path_to_your_fgdata

We could probably make the 2.10.0 version run, but I had already
upgraded it to 2.12.0 before I made it work. It seems only the data dir
configuration was wrong/missing: FG_DATA_DIR="$out/share/FlightGear/".

Simgear must be in version sync with flightgear, hence the upgrade.

Most of the diff is because I rewrote the flightgear expression to what
I think is the "new style". I wanted to add a preConfigure phase but
didn't understand why this expression had custom phases. So I just
rewrote the whole thing to something I consider more readable.
update dwb to the latest revision
Add `usb` haskell package
experimental/vsftpd

vsftpd improvements:
- intorduce one declarative list of options
- make docummentation strings more understandable and add missing options
  such as SSL/TLS support
- Use environment.etc."vsftpd".text because I can't think about any
  reason why a shell script should be used.
  That code was written in 2009.
nixos/graphite: listen on localhost by default
nixos/statsd: Add support to specify managment ip/port
nixos/elasticsearch: change loglevel from DEBUG to INFO
Adding tox-prpl, a pidgin plugin for tox.

I use the current master, because the release 0.2.0 doesn't match the
libtoxcore expression.
Fixes for torprpl, until making it work.
Updating bitcoin to 0.8.5
libav: maintenance update 9.9 -> 9.10
Merge pull request #1068 from offlinehacker/nixos/elasticsearch/logfix

nixos/elasticsearch: change loglevel from DEBUG to INFO