MFC r215166,215377,215391,215392,215393,215395,216101,216103,216105,216107,

MFC r215166,215377,215391,215392,215393,215395,216101,216103,216105,216107,
    216749,216760,217748,218167:

- Add a KPI and supporting infrastructure to allow modular congestion control
  algorithms to be used in the net stack. Algorithms can maintain per-connection
  state if required, and connections maintain their own algorithm pointer, which
  allows different connections to concurrently use different algorithms. The
  TCP_CONGESTION socket option can be used with getsockopt()/setsockopt() to
  programmatically query or change the congestion control algorithm respectively
  from within an application at runtime.

- Integrate the framework with the TCP stack in as least intrusive a manner as
  possible. Care was also taken to develop the framework in a way that should
  allow integration with other congestion aware transport protocols (e.g.  SCTP)
  in the future. The hope is that we will one day be able to share a single set
  of congestion control algorithm modules between all congestion aware transport
  protocols.

- Introduce a new congestion recovery (TF_CONGRECOVERY) state into the TCP stack
  and use it to decouple the meaning of recovery from a congestion event and
  recovery from packet loss (TF_FASTRECOVERY) a la RFC2581. ECN and delay based
  congestion control protocols don't generally need to recover from packet loss
  and need a different way to note a congestion recovery episode within the
  stack.

- Remove the net.inet.tcp.newreno sysctl, which simplifies some portions of code
  and ensures the stack always uses the appropriate mechanisms for recovering
  from packet loss during a congestion recovery episode.

- Extract the NewReno congestion control algorithm from the TCP stack and
  massage it into module form. NewReno is always built into the kernel and will
  remain the default algorithm for the forseeable future. Implementations of
  additional different algorithms will become available in the near future.

- 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 802504.

In collaboration with:	David Hayes <dahayes at swin edu au> and
				Grenville Armitage <garmitage at swin edu au>
Sponsored by:	Cisco URP, FreeBSD Foundation
Reviewed by:	rpaulo (r215166), bz (r215391,215395,216749,217748)
Tested by:	David Hayes (r215166), trociny (r215377,215391,215392,215395)

git-svn-id: svn://svn.freebsd.org/base/stable/8@222401 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f