Split the QUEUE_MACRO_DEBUG into QUEUE_MACRO_DEBUG_TRACE and
QUEUE_MACRO_DEBUG_TRASH.
Add the debug macrso QMD_IS_TRASHED() and QMD_SLIST_CHECK_PREVPTR().
Document these in queue.3.
Reviewed by: emaste
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D3984