MFC r216758,217252:

MFC r216758,217252:

- Add some helper hook points to the TCP stack. The hooks allow Khelp modules to
  access inbound/outbound events and associated data for established TCP
  connections. The hooks only run if at least one hook function is registered
  for the hook point, ensuring the impact on the stack is effectively nil when
  no TCP Khelp modules are loaded. struct tcp_hhook_data is passed as contextual
  data to any registered Khelp module hook functions.

- Add an OSD (Object Specific Data) pointer to struct tcpcb to allow Khelp
  modules to associate per-connection data with the TCP control block.

- Tweak the MFCed code to preserve the ABI of the 8-STABLE branch with respect
  to "struct tcpcb" by consuming some of the padding within the struct.

- Bump __FreeBSD_version to 802506.

In collaboration with:	David Hayes <dahayes at swin edu au> and
				Grenville Armitage <garmitage at swin edu au>
Sponsored by:	FreeBSD Foundation
Reviewed by:	bz, others along the way

git-svn-id: svn://svn.freebsd.org/base/stable/8@222408 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f