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).
nixos/nginx: allow to specify which package to use

This enables to override which plugins to include with nginx by allowing to change
nginx package. I also removed webdav option in nginx nixos module, because you
can now specify this by overriding nginx package.
php54: add pcntl option
Attempt at compiling MilkyTracker, but recent zlib version may have broken it.

There's a zlib version included with milkytracker,
but there's no makefiles for it. I've only included
the header here, but it fails at link-time with
several 'undefined reference' errors, which simply
means it can't find the definitions, e.g. compiled
zlib.
There's bug reports on other package systems although
unfortunately still unresolved.
https://bugs.archlinux.org/task/31324
http://lists.freebsd.org/pipermail/freebsd-ports/2013-March/082180.html
MilkyTracker: fix build, use system zlib and decompressor patch

Also add JACK support
MilkyTracker: Tidy up, sort buildInputs and use the attrPath for the license
Clean up Synergy option descriptions a bit
Remove remaining references to Upstart
Remove unused file
Manual: Mention other ways to get NixOS
Manual: Document how to use NixOS channels
Merge pull request #1132 from lovek323/php54-pcntl

php54: add pcntl option
Document the NixOS configuration syntax

...without telling people to read the Nix manual first.
Fix mkForce example
Manual tweaks
Manual: Don't tell users to delete ~/.nix-defexpr/channels

That breaks nix-channel, obviously.
Propagate the stableBranch argument
Manual: Expand the Development chapter
Loosen the type of SSH key files
Mark the value for nixpkgs.system as a default