shadow: fix the behavior of su

shadow: fix the behavior of su

In systems where bash is not installed and /bin/sh is provided by
busybox. Commands like `su -l -c '/home/root/test' xuser' would fail
complaining the the 'su' applet could not be found.

This patch references the old version of shadow to keep the behaviour
the way it was in old version so that we would avoid the problem mentioned
above.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>