Allow ECN to be accepted for incoming TCP connections without requesting it for outgoing connections and make this the default

[truckman]

Summary

Solaris and Linux (since 2.4.20) have allowed three settings for
ECN for some time.

• Totally disabled
• Enabled for incoming TCP connections if requested, but not

	    requested by outgoing TCP connections (Linux default)
	  * Enabled for incoming TCP connections if requested and
	    requested by outgoing TCP connections

Mac OS X has had independent knobs for incoming and outgoing
ECN support since 10.5. As of OS X, 10.11, Apple claims that
ECN is enabled by default, though it not clear whether that
is for both incoming and outgoing connections.

This patch changes the FreeBSD net.inet.tcp.ecn.enable knob
from binary off/on to a three way setting to match Linux
and Solaris. The values were chosen to match Linux.

	  0 - Totally disabled
	  1 - Enabled for both incoming and outgoing
	  2 - Enabled for incoming if requested, and outgoing connections
	      do not request ECN

with 2 being the new default. For hosts currently running with
net.inet.tcp.ecn.enable=1, there should be no change in
behavior.

Test Plan

Test incoming and outgoing TCP connections with various settings
of the net.inet.tcp.ecn.enable knob