Strengthened test and fixed a bug on dlist.
The test acceptance criteria are now stricter with white- and black-box checks added. New white-box checking, borrowed from alarm's linked-list verify ensures that headed lists reach last when traversed starting from first, and vice-versa. New black-box checking ensures the link from an ending element to the head sentinel is intact.
The bug was that element insertion at an end of a headed list broke the new black-box criterion. The new white-box criterion was never violated in the current test cases, but early work at integrating this list with the alarm system exercised such a break.
Both checks behind the new test criteria are packaged as a verify routine for user code to invoke.
