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/tools/networking/chrony/default.nix

@@ -13,7 +13,7 @@ stdenv.mkDerivation rec {
   buildInputs = [ readline ] ++ stdenv.lib.optional stdenv.isLinux libcap;
 
   meta = with stdenv.lib; {
-    description = "A pair of programs which are used to maintain the accuracy of the system clock on a computer.";
+    description = "Sets your computer's clock from time servers on the Net";
     homepage = "http://chrony.tuxfamily.org/";
     license = licenses.gpl2;
     platforms = platforms.unix;