Deprecated switch function to if/elsif.  Added a section in the GETNEXT ...

...portion to properly handle requests for OIDs without the trailing .0

I added some more detailed comments below.  This is a very good script, it just would not work with my version of Perl since I didn't have the 'switch' feature.

Also, my application OpenNMS was expecting slightly different functionality when requesting an OID without the trailing .0 index.