Correctly unlink item from work list.  The old code could attempt
to unlink from the work list using using clink. This would lead to
an assertion failure as the item had just been correctly unlinked
from connecting using clink.

Allow dead servers to be summarized as "all timeout".
Allow lame servers to be summarized as "all servfail" or "all refused".

For servers where the zone isn't loaded, the test should be seen OPT
and "all servfail" or "all refused" for version=0, not "all servfail"
and "all refused".

Resolve "report containing zone for no NS records if possible"

Closes #30

See merge request !15