x/crypto/ssh: Add protocol version to ServerVersion
x/crypto/ssh: Add protocol version to ServerVersion

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 <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>

bford/golang-x-crypto,mattharden/crypto,golang/crypto,cmars/tools,bford/golang-x-crypto,kim-racktop/crypto,pki-io/crypto,golang/crypto,pki-io/crypto,cmars/tools,kim-racktop/crypto,mattharden/crypto,ScriptRock/crypto,thomaso-mirodin/crypto,cmars/tools,cmars/tools,thomaso-mirodin/crypto,ScriptRock/crypto,golang/crypto