PreferenceTool: Get acl_users by acquisition.

acl_users is one of the rare objects (with SQL connectors) which are
intended to be fetched by acquisition, allowing overrides.
Also, move code out of try..finally block where possible.

product/ERP5Form/PreferenceTool.py

@@ -253,12 +253,12 @@ class PreferenceTool(BaseTool):
     """Creates a preference for a given user, and optionnally enable the
     preference.
     """
-    security_manager = getSecurityManager()
-    try:
-      user_folder = self.getPortalObject().acl_users
+    user_folder = self.acl_users
     user = user_folder.getUserById(username)
     if user is None:
       raise ValueError("User %r not found" % (username, ))
+    security_manager = getSecurityManager()
+    try:
       newSecurityManager(None, user.__of__(user_folder))
       preference = self.newContent(portal_type='Preference')
       if enable: