x/crypto/ssh: fix host certificate principal evaluation to check for hostname only

SSH host certificates are expected to contain hostnames only,
not "host:port" format.

This change allows Go clients to connect to OpenSSH servers that
use host certificates.

Note, this change will break any clients that use ssh.NewClientConn()
with an `addr` that is not in `host:port` format (they will see a
"missing port in address" error).

Fixes bug 20273.

Change-Id: I5a306c6b7b419a737e1f0f9c5ca8c585e21a45a4
Reviewed-on: https://go-review.googlesource.com/43475
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
5 files changed