sysctl: terminate strings also on \r

This partially mitigates a common strategy used by attackers for hiding
the full contents of strings in procfs from naive sysadmins who use cat,
more or sysctl to inspect the contents of strings in procfs.

References: http://www.jakoblell.com/blog/2014/05/07/hacking-contest-hiding-stuff-from-the-terminal/
Signed-off-by: Paul Wise <pabs3@bonedaddy.net>
Signed-off-by: Kees Cook <keescook@chromium.org>