queue(3): Enhance queue debugging macros

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