Use anonymous unions to add possibility to put mbufs into queue(3)
STAILQs and SLISTs using the same structure field as good old m_next
and m_nextpkt linkage occupy.

New code is encouraged to use queue(3) macros, instead of implementing
the wheel. However, better not to have a mixture of old style and
queue(3) in one file or subsystem.

Reviewed by: rwatson, rrs, rpaulo
Differential Revision: D1499