Numerous cleanups for snmp_enumusers

* Bring in line with Ruby standards
* More sane format for adding new OSs
* Better logging for use on larger networks
* Better error handling

See my validation in #3516

/cc @jvennix-r7 -- given the age of #3516, if @midnitesnake doesn't respond here, I could push the equivalent of this PR when I land #3516 if you could approve here