insane.bbclass: make the checking stricter for unsafe references in scripts

insane.bbclass: make the checking stricter for unsafe references in scripts

Previously, the checking for unsafe references is not strict enough. It
only checks whether '/usr/' is in the script. As a result, any script
containing statements like below will match this check.

	   PATH="/bin:/sbin:/usr/bin:/usr/sbin"

However, as we can see, this is actually not an unsafe reference. What
we really want to check is something like '/usr/bin/tail', so we should
make the checking stricter.

This patch solves the QA warning in gzip and nfs-utils.

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