More description fixes

* Remove package name
* Start with upper case letter
* Remove trailing period

Also reword some descriptions and move some long descriptions to
longDescription.

I'm not touching generated packages.

pkgs/development/libraries/libnet/default.nix

@@ -11,7 +11,7 @@ stdenv.mkDerivation rec {
 
   meta = {
     homepage = http://github.com/sam-github/libnet;
-    description = "Libnet provides a portable framework for low-level network packet construction.";
+    description = "Portable framework for low-level network packet construction";
     license = stdenv.lib.licenses.bsd3;
     platforms = stdenv.lib.platforms.unix;
   };