ssh: add debug print at the lowest level

This is a simple minded, fast print, suitable for debugging timing
sensitive issues.

Change-Id: I9ae3df5fe86f1883c1fa9265b6f7f9a98d33747e
Reviewed-on: https://go-review.googlesource.com/36054
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>