Rework examples/snmpd.conf; show more things that people are likely to

Rework examples/snmpd.conf; show more things that people are likely to
actually need, and fewer things that they won't need. In particular don't
rely on default auth/enc settings; SNMPv3 isn't algorithm-agile, it must
be defined on both ends, so relying on a default is going to cause some
problems if it ever does change.