Add system option for ctrl-alt-del-action to configuration

This new configuration option allows the user to specify
what action to take on ctrl-alt-delete: disable, reboot
or poweroff. By default ctrl-alt-delete-action is set
to disable.

The ctrl-alt-delete action is configured in /etc/inittab.
Use elif conditional instead of if
Add syntax check to validate value
Merge pull request #9 from hydrajump/ctrl-alt-del-action

Bug #173: Add system option for ctrl-alt-del-action to configuration