x/crypto/ssh: set constraints when adding certs to the agent Fixes golang/go#15953 Change-Id: Ia36b5422bef14609d512c3f5055a3bffad18ce0f Reviewed-on: https://go-review.googlesource.com/23752 Reviewed-by: Han-Wen Nienhuys <hanwen@google.com> Run-TryBot: Han-Wen Nienhuys <hanwen@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org>