Add support for SAnon, a simple key agreement protocol that provides no
authentication of initiator or acceptor using x25519 ECDH key exchange.
See doc/standardization/draft-howard-gss-sanon-xx.txt for a protocol
description.
The X25519 implementation comes from libsodium. Explicit copyright
notices have been added to each file as well as some portability changes
(e.g. align.h).