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/development/libraries/avahi/default.nix

@@ -38,7 +38,7 @@ stdenv.mkDerivation rec {
   '';
 
   meta = with stdenv.lib; {
-    description = "Avahi, an mDNS/DNS-SD implementation";
+    description = "mDNS/DNS-SD implementation";
     homepage    = http://avahi.org;
     license     = licenses.lgpl2Plus;
     platforms   = platforms.unix;