MFC r300240

MFC r300240

Change net.inet.tcp.ecn.enable sysctl mib from a binary off/on
control to a three way setting.
  0 - Totally disable ECN. (no change)
  1 - Enable ECN if incoming connections request it.  Outgoing
      connections will request ECN.  (no change from present != 0 setting)
  2 - Enable ECN if incoming connections request it.  Outgoing
      conections will not request ECN.

Change the default value of net.inet.tcp.ecn.enable from 0 to 2.

Linux version 2.4.20 and newer, Solaris, and Mac OS X 10.5 and newer have
similar capabilities.  The actual values above match Linux, and the default
matches the current Linux default.

Reviewed by:	eadler
Relnotes:	yes
Differential Revision:	https://reviews.freebsd.org/D6386

git-svn-id: svn://svn.freebsd.org/base/stable/10@302032 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f