fix(libtofs): “files_switch” mess up the variable defined by “map.jinja”

- “map.jinja” export a variable name as “<tplroot>”
- import of “libtofs.jinja” is done with context and has access to
  “<tplroot>” variable
- in “files_switch”, appending the empty string to the “fsl” variable
  modify globally “<tplroot>['tofs']['files_switch']”

* TEMPLATE/libtofs.jinja: initialise “fsl” with a copy of
  “files_switch_list” instead of a reference.

TEMPLATE/libtofs.jinja

@@ -78,7 +78,7 @@
     {#- Use the default, new method otherwise #}
     {%- set fsl = salt['config.get'](
         tplroot ~ path_prefix_ext|replace('/', ':') ~ ':files_switch',
-        files_switch_list
+        files_switch_list[:]
     ) %}
     {#- Append an empty value to evaluate as `default` in the loop below #}
     {%- if '' not in fsl %}