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/tools/security/tboot/default.nix

@@ -21,9 +21,7 @@ stdenv.mkDerivation rec {
   installFlags = "DESTDIR=$(out)";
 
   meta = with stdenv.lib; {
-    description = ''Trusted Boot (tboot) is an open source, pre-kernel/VMM module that uses
-                    Intel(R) Trusted Execution Technology (Intel(R) TXT) to perform a measured
-                    and verified launch of an OS kernel/VMM.'';
+    description = "A pre-kernel/VMM module that uses Intel(R) TXT to perform a measured and verified launch of an OS kernel/VMM";
     homepage    = http://sourceforge.net/projects/tboot/;
     license     = licenses.bsd3;
     maintainers = [ maintainers.ak ];