Backported from X.org:

commit 5c2e2a164d615ab06be28a663734e782614b5cc7
Author: Daniel Kurtz <djkurtz@chromium.org>
Date:   Wed Apr 18 09:51:51 2012 +0000

    os/xprintf: add Xvscnprintf and Xscnprintf

    Normal snprintf() usually returns the number of bytes that would have been
    written into a buffer had the buffer been long enough.

    The scnprintf() variants return the actual number of bytes written,
    excluding the trailing '\0'.
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Backported-to-NX-by: Mihai Moldovan <ionic@ionic.de>