Fix many package descriptions

(My OCD kicked in today...)

Remove repeated package names, capitalize first word, remove trailing
periods and move overlong descriptions to longDescription.

I also simplified some descriptions as well, when they were particularly
long or technical, often based on Arch Linux' package descriptions.

I've tried to stay away from generated expressions (and I think I
succeeded).

Some specifics worth mentioning:
 * cron, has "Vixie Cron" in its description. The "Vixie" part is not
   mentioned anywhere else. I kept it in a parenthesis at the end of the
   description.

 * ctags description started with "Exuberant Ctags ...", and the
   "exuberant" part is not mentioned elsewhere. Kept it in a parenthesis
   at the end of description.

 * nix has the description "The Nix Deployment System". Since that
   doesn't really say much what it is/does (especially after removing
   the package name!), I changed that to "Powerful package manager that
   makes package management reliable and reproducible" (borrowed from
   nixos.org).

 * Tons of "GNU Foo, Foo is a [the important bits]" descriptions
   is changed to just [the important bits]. If the package name doesn't
   contain GNU I don't think it's needed to say it in the description
   either.

pkgs/development/libraries/talloc/default.nix

@@ -16,7 +16,7 @@ stdenv.mkDerivation rec {
   '' else "";
 
   meta = {
-    description = "talloc is a hierarchical pool based memory allocator with destructors";
+    description = "Hierarchical pool based memory allocator with destructors";
     homepage = http://tdb.samba.org/;
     license = stdenv.lib.licenses.gpl3;
     platforms = stdenv.lib.platforms.all;