prefer central root: just one copy of this bulky material;

--- a/etc/settings	Thu Mar 21 12:38:06 2019 +0100
+++ b/etc/settings	Thu Mar 21 12:41:58 2019 +0100
@@ -142,7 +142,7 @@
 ### OCaml
 ###
 
-ISABELLE_OPAM_ROOT="$ISABELLE_HOME_USER/opam"
+ISABELLE_OPAM_ROOT="$USER_HOME/.opam"
 
 ISABELLE_OCAML_VERSION="4.05.0"
 
@@ -151,7 +151,7 @@
 ### Haskell
 ###
 
-ISABELLE_STACK_ROOT="$ISABELLE_HOME_USER/stack"
+ISABELLE_STACK_ROOT="$USER_HOME/.stack"
 
 ISABELLE_STACK_RESOLVER="lts-12.25"