This change implements support for extended echo request and reply
messages, and interface identification object that can be used to test
the status of a probed interface via a proxy interface from a probing
interface as defined in RFC 8335.

It's package user's responsibility to make a complete RFC-compliant
PROBE initiator implementation using ipv4, ipv6 and icmp packages of
x/net repository.

Fixes golang/go#24440.

Change-Id: I87ab8a7581c4d41a0c579805b0e3043e48ac85f0
Reviewed-on: https://go-review.googlesource.com/63999Reviewed-by: Ian Lance Taylor <iant@golang.org>