In ddb(4) print mbuf chain and packet list.

In ddb(4) print mbuf chain and packet list.

For debugging hardware offloading, DMA requirements, bounce buffers,
and performance optimizations, knowing the memory layout of mbuf
content helps.
Implement /c and /p modifiers in ddb show mbuf.  It traverses the
pointer m_next for mbuf chain or m_nextpkt for packet list.  Show
mbuf type, data offset, mbuf length, packet length, cluster size,
and total number of elements, length and size.

OK claudio@ mvs@