Run /opt/shutdown.sh from init's rc.shutdown

Modify /etc/init.d/rc.shutdown during iso build to call /opt/shutdown.sh
first.  This ensures /opt/shutdown.sh runs when user executes
/sbin/poweroff or /sbin/reboot.

Signed-off-by: Justin Charette <charetjc@gmail.com>
Replace exitcheck.sh with poweroff in ACPI power event

If exitcheck.sh is left in ACPI, then the /opt/shutdown.sh will run
twice; Once from exitcheck.sh, and again from /etc/init.d/rc.shutdown.

Signed-off-by: Justin Charette <charetjc@gmail.com>
Inform user when shutdown script runs during init shutdown/reboot

Signed-off-by: Justin Charette <charetjc@gmail.com>