280be00 introduced custom server versions; however, at least OpenSSH
clients do not accept a server version that doesn't start with a
protocol version like "SSH-2.0-"; this is not documented in this
library, and automatically adding it in case that the user did not does
no harm.
Change-Id: Iafc23b23160a8277b10cbf29778dc64a96351bd7
Reviewed-on: https://go-review.googlesource.com/14018
Reviewed-by: Adam Langley <agl@golang.org>
