traceroute6: Implement ECN bleaching detection

Explicit Congestion Notification (ECN) is a mechanism that allows
end-to-end notification of network congestion without dropping packets
by explicitly setting the ECN code point (2 bits).

Per RFC 8087, section 3.5, network devices should not be configured to
change the ECN code point in the packets that they forward, except to
set the CE (Congestion Experienced) code point ('11') to signal
incipient congestion.

The current commit adds an -E flag to traceroute6 that crafts a packet
with an ECT(1) code point ('01').

If the packet is received back with a zero ECN code point ('00'), it
outputs that the hop in question erases or "bleaches" the ECN code point
values. Bleaching may occur for various reasons (including normalizing
packets to hide which equipment supports ECN). This policy prevents the
use of ECN by applications.

If the packet is received back with an all-ones ECN code point ('11'),
it outputs that the hop in question is experiencing "congestion".

If the packet is received back with a different ECN code point ('10'),
it outputs that the hop in question changes or "mangles" the ECN code
point values.

If the packet is received with the same ECN code point that was sent
('01'), it outputs that the hop has "passed" the ECN bits appropriately.

Inspired by: Darwin
Reviewed by: imp, markj
MFC after: 1 month
Pull Request: https://github.com/freebsd/freebsd-src/pull/879

(cherry picked from commit 0c2218d1d5fda2c579d3d33f1fd3af9ad447e160)