numlockx: Add meta.mainProgram and clean up

pkgs/tools/X11/numlockx/default.nix

···
-{ lib, stdenv, fetchFromGitHub, libX11, libXext, autoconf }:
+{ lib, stdenv, fetchFromGitHub, autoconf, libX11, libXext }:
 
-stdenv.mkDerivation rec {
+stdenv.mkDerivation {
   version = "1.2";
   pname = "numlockx";
 
   src = fetchFromGitHub {
     owner = "rg3";
-    repo = pname;
+    repo = "numlockx";
     rev = "9159fd3c5717c595dadfcb33b380a85c88406185";
-    sha256 = "1w49fayhwzn5rx0z1q2lrvm7z8jrd34lgb89p853a024bixc3cf2";
+    hash = "sha256-wrHBelxEADUKugmtR8loWaJ/6s5U4PBBz8V+Dr1yifA=";
   };
 
   buildInputs = [ libX11 libXext autoconf ];
 
-  meta = with lib; {
+  meta = {
     description = "Allows to start X with NumLock turned on";
-    license = licenses.mit;
-    platforms = platforms.all;
+    license = lib.licenses.mit;
+    platforms = lib.platforms.all;
+    mainProgram = "numlockx";
   };
 }