+JH/15 Disallow tainted metadata in lists.
+ - Change-of-separator prefixes are handled specially when they are
+ explicit text; only the remainder of the list is expanded. A change-of-
+ separator resulting from expansion will not take effect if tainted.
+ - Elements starting with a plus-sign (named-list inclusion,
+ case-interpretation etc) and (hostlist) @[] (et al) are not handled
+ specially and are still operative at this time - but warnings are logged;
+ if any of these are needed in a list with a tainted element (which taints
+ the entire list at string-expansion time) then a named-list can be used
+ for that element.
+ - Exclamation-marks ("!" signifying negation) are not checked for taint
+ at this time.
+