MFp4 bz_ipv6_fast:

MFp4 bz_ipv6_fast:

  in_cksum.h required ip.h to be included for struct ip.  To be
  able to use some general checksum functions like in_addword()
  in a non-IPv4 context, limit the (also exported to user space)
  IPv4 specific functions to the times, when the ip.h header is
  present and IPVERSION is defined (to 4).

  We should consider more general checksum (updating) functions
  to also allow easier incremental checksum updates in the L3/4
  stack and firewalls, as well as ponder further requirements by
  certain NIC drivers needing slightly different pseudo values
  in offloading cases.  Thinking in terms of a better "library".

  Sponsored by:	The FreeBSD Foundation
  Sponsored by:	iXsystems

Reviewed by:	gnn (as part of the whole)
MFC After:	3 days