datatypes, not only strings.  parse_hook is useless for bool, I suppose,
but it seems possibly useful for int and double to apply variable-specific
constraints that are more complex than simple range limits.  assign_hook
is definitely useful for all datatypes --- we need it right now for bool
to support date cache reset when changing Australian timezone rule setting.
Also, clean up some residual problems with the reset all/show all patch,
including memory leaks and mistaken reset of PostPortNumber.  It seems
best that RESET ALL not touch variables that don't have SUSET or
USERSET context.