x/crypto/ssh: implement curve25519-sha256@libssh.org key agreement.

Fixes golang/go#11004.

Change-Id: Ic37cf9d620e3397b7ad769ae16abdaee63a7733b
Reviewed-on: https://go-review.googlesource.com/13592
Reviewed-by: Adam Langley <agl@golang.org>
2 files changed