Non-functional cleanup of ng_ksocket_incoming2().
Whitespace.
When soreceive_generic() hands off an mbuf from buffer,
clear its pointer to next record, since next record
belongs to the buffer, and shouldn't be leaked.

The ng_ksocket(4) used to clear this pointer itself,
but the correct place is here.

Sponsored by:	Nginx, Inc
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
Once ng_ksocket(4) is fixed, re-apply r194662. See this revision for
longer description.

Discussed with:	andre, rwatson
Sponsored by:	Nginx, Inc.
Fix bug in m_split() in a case when split len matches len of the
first mbuf, and the first mbuf is M_PKTHDR.

PR:		kern/176144
Submitted by:	Jacques Fourie <jacques.fourie gmail.com>