chromium: Check sha256 per channel in update.sh.

Until this commit we had a single hash of the whole version response from
omahaproxy. This worked well for not updating unnecessarily but only until one
single channel has a new version available.

Unfortunately the dev version is released quite frequently, so the hash is of no
use at all (we could rather directly fetch everything everytime we execute the
script).

This led to this commit, which adds two methods of caching:

First of all, if a perticular version/channel is already in the previous version
of the sources.nix file, don't download it again.

And the second method is to check if the current sha256 is already downloaded
and reads the corresponding sha256 from the lookup table.

So, this should really help to avoid flooding the download servers and to not
stress impatient users too much.
chromium: Update channels to latest versions.

This removes the now obsolete VHASH line in sources.nix aswell.
nix: Update to 1.2pre2793_d950cfe

Necessary for systemd.
man-pages: Update to 3.42
systemd: Set the path to pkttyagent

This allows commands like "systemctl reboot" to ask for authentication
when run by mere mortals.
upower: Update to 0.9.18 and enable systemd support
polkit: Update to 0.105 and enable systemd (logind) support

There are even newer polkit releases, but they have a spidermonkey
dependency which I don't want to bother with right now.
postgresql: Update to latest versions

Fixes CVE-2012-3488, CVE-2012-3489.
stdenv: Add `isGlibc' function.
libiconvOrNull: Do not use an external libiconv on glibc systems.
stdenv: Fix typo.
glib: Use `libiconvOrNull' instead of `libiconv'.
libiconvOrNull: Fix libc check.
gdb: Update to 7.5
Merge branch 'master' of github.com:NixOS/nixpkgs
Add latest patchelf prerelease
nss: Update NSS to version 3.13.6.

So to begin with fixing NSS let's get to the latest upstream release and start
fixing, so we won't carry around historic crap we then will throw away anyway.
nss: Build using system libsqlite.

Let's use system SQLite library, which makes sense anyway. More importantly
because it conflicts with the sqlite package, as NSS is building this as a
shared library aswell.
nss: Add nss-pem module from fedora.

This is a compatibility module which adds suport for PEM certificates used by
OpenSSL and compatible libraries. The module gets built but isn't used at the
moment, so we're going to work on integration of it later.
nss: Clean up build/make flags.

First of all, let's remove that redundant BUILD_OPT variable.

This variable already is in makeFlags, so we really don't want it to be lurking
around in the attribute set of the derivation, and it annoys me for being there
for days.

We now state build targets explicitly rather than relying on "nss_build_all".
This makes NSPR_CONFIG_STATUS and the touch of build_nspr stamp obsolete, as
only nss_build_all includes build_nspr.

In addition, we don't need the -lz hack anymore, as this has been fixed in
recent NSS versions, so we can completly remove the postBuild hook.

And while we're at it, we're removing those outdated build instructions as well,
especially because we don't and can't follow official building guidelines
anymore, as those are difficult to apply to Nix.