add multi-drive support

It's a big one.  If the -d option is not specified, it does a scan for all devices and adds them to the list to inspect.  smartctl is run for each device and the results are aggregated (critical for one drive makes overall status critical).  Each drive outputs a summary of status on a separate line and the result looks reasonable in the nagios web console.

This would benefit from a little more refactoring for consistency, but the kids woke up.  If you agree on direction, I'll continue next week.