net-mgmt/net-snmp: Avoid changing to the snmpd user for now

Switching the user might break existing configurations, for reasons that
are not fully clear yet. Avoid surprises by letting users opt-in to
that change, for now.

Also provide an rc.conf variable to make it easy to configure snmpd to
drop privileges if they so choose.

Reported by: dvl
Approved by: zi
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D45199