Some description fixes

There are many more packages to fix, this is just a start.

Rules:
 * Don't repeat the package name (not always that easy...)
 * Start with capital letter
 * Don't end with full stop
 * Don't start with "The ..." or "A ..."

I've also added descriptions to some packages and rewritten others.

pkgs/os-specific/linux/apparmor/default.nix

@@ -85,7 +85,7 @@ stdenv.mkDerivation rec {
 
   meta = with stdenv.lib; {
     homepage = http://apparmor.net/;
-    description = "A Linux application security system";
+    description = "Linux application security system";
     license = licenses.gpl2;
     maintainers = [ maintainers.phreedom ];
     platforms = platforms.linux;