su: Make the su package a provider of only the su binary

Additionally, provide su with the base system and remove su from the
util-linux package as it is now provided by shadow.

pkgs/os-specific/linux/shadow/default.nix

@@ -26,6 +26,8 @@ stdenv.mkDerivation rec {
 
   patches = [ ./keep-path.patch dots_in_usernames ];
 
+  outputs = [ "out" "su" ];
+
   # Assume System V `setpgrp (void)', which is the default on GNU variants
   # (`AC_FUNC_SETPGRP' is not cross-compilation capable.)
   preConfigure = "export ac_cv_func_setpgrp_void=yes";
@@ -35,10 +37,14 @@ stdenv.mkDerivation rec {
       substituteInPlace lib/nscd.c --replace /usr/sbin/nscd ${glibc}/sbin/nscd
     '';
 
-  # Don't install ‘groups’, since coreutils already provides it.
   postInstall =
     ''
+      # Don't install ‘groups’, since coreutils already provides it.
       rm $out/bin/groups $out/share/man/man1/groups.*
+
+      # Move the su binary into the su package
+      mkdir -p $su/bin
+      mv $out/bin/su $su/bin
     '';
 
   meta = {