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/security/pass/default.nix

@@ -74,8 +74,8 @@ in stdenv.mkDerivation rec {
     tree
     which
     qrencode
+    procps
   ] ++ optional tombPluginSupport tomb
-    ++ optional stdenv.isLinux procps
     ++ ifEnable x11Support [ dmenu xclip xdotool ]);
 
   postFixup = ''