unix-tools: introduce unix-tools.nix

unix-tools.nix has a collection of tools that are commonly installed
by default in Unix derivatives. This is intended to provide
compatibility between macOS and Linux users. Three Linux-only
derivations are provided for compatbility:

- procps
- utillinux
- nettools

More tools are also provided.

Also: treewide: use unixtools

Non-comprehensive replace of Linux-only procps and util-linux with
'unixtools'.

pkgs/tools/misc/parallel/default.nix

@@ -12,7 +12,7 @@ stdenv.mkDerivation rec {
 
   postInstall = ''
     wrapProgram $out/bin/parallel \
-      ${if stdenv.isLinux then ("--prefix PATH \":\" ${procps}/bin") else ""} \
+      --prefix PATH : "${procps}/bin" \
       --prefix PATH : "${perl}/bin" \
   '';