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>
2 files changed