Revamp mbuf handling in ng_ksocket_incoming2():
Revamp mbuf handling in ng_ksocket_incoming2():

- Clear code that workarounded a bug in FreeBSD 3,
  and even predated import of netgraph(4).
- Clear workaround for m_nextpkt pointing into
  next record in buffer (fixed in r248884).
  Assert that m_nextpkt is clear.
- Do not rely on SOCK_STREAM sockets containing
  M_PKTHDR mbufs. Create a header ourselves and
  attach chain to it. This is correct fix for
  kern/154676.

PR:		kern/154676
Sponsored by:	Nginx, Inc

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