changing net.inet.tcp.ecn.enable=1 to a three-way knob

[Don Lewis]

I posted a patch here: <https://reviews.freebsd.org/D6386> to change
net.inet.tcp.ecn.enable from a binary off/on knob to a three way knob
that adds a setting allow incoming TCP connections to negotiate ECN
while outgoing connections don't request ECN.  Always requesting ECN on
outgoing connections can be troublesome because of the number of broken
middle-boxes that block ECN connections (though we do have a fallback to
turning off ECN if the initial connection request fails).

I'd like to get this into HEAD before the code slush.