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/tools/networking/ratools/default.nix

@@ -23,7 +23,7 @@ stdenv.mkDerivation rec {
   '';
 
   meta = with stdenv.lib; {
-    description = "a fast, dynamic, multi-threading framework for IPv6 Router Advertisements";
+    description = "A fast, dynamic, multi-threading framework for IPv6 Router Advertisements";
     homepage = https://github.com/danrl/ratools;
     license = licenses.asl20;
     platforms = platforms.linux;