List Flag: loop-detect

Causes distributed items (submissions to lists and digests) to have an
additional header added called "X-Loop-Detect:" which contains the hostname
of the machine running listmanager and the name of the mailing list to
which the item was distributed.  Listmanager watches for this header and,
if it is discovered, assumes that the submission has somehow been addressed
back to the list and discards it to prevent the same posting from being
sent to the list repeatedly.