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