Captialize meta.description of all packages

In line with the Nixpkgs manual.

A mechanical change, done with this command:

  find pkgs -name "*.nix" | \
      while read f; do \
          sed -e 's/description\s*=\s*"\([a-z]\)/description = "\u\1/' -i "$f"; \
      done

I manually skipped some:

* Descriptions starting with an abbreviation, a user name or package name
* Frequently generated expressions (haskell-packages.nix)

pkgs/misc/screensavers/light-locker/default.nix

@@ -31,7 +31,7 @@ stdenv.mkDerivation rec {
 
   meta = with stdenv.lib; {
     homepage = https://github.com/the-cavalry/light-locker;
-    description = "light-locker is a simple locker";
+    description = "Light-locker is a simple locker";
     longDescription = ''
       light-locker is a simple locker (forked from gnome-screensaver) that aims to have simple, sane, secure defaults and be well integrated with the desktop while not carrying any desktop-specific dependencies.
       It relies on lightdm for locking and unlocking your session via ConsoleKit/UPower or logind/systemd.