checksec: Improvements suggested by Eelco

Signed-off-by: Austin Seipp <aseipp@pobox.com>
checksec: ensure readelf is installed.

Also ensure checksec always uses the correct executable.

Signed-off-by: Austin Seipp <aseipp@pobox.com>
checksec: fix description.

I accidentally copy pasta'd.

Signed-off-by: Austin Seipp <aseipp@pobox.com>