Tighten default security for snmpd(8). This is done by doing several

Tighten default security for snmpd(8). This is done by doing several
things:
- Only allow SNMPv3 by default. SNMPv1 and SNMPv2c can be enabled by
  setting the new snmpv* flags on the "liston on" statements.
- Remove the default community names. They're not secure to use.
- Change the default seclevel to enc.

Initial idea, help from and OK sthen@