net-mgmt/net-snmp: fix net-snmp-config on jail hosts

re https://github.com/net-snmp/net-snmp/issues/280

This patch allows 'net-snmp-config --create-snmpv3-user' to succeed
if run on a jail host and snmpd is also running in one of the jails.

Thhis patch limits the check to the host, ingnoring jails.

PR: 269791