net-mgmt/cacti: update cacti config to use net-smtp by default

Latest php upgrade introduces an issue between cacti and php-snmp
causing data retrieval issues. This not occurs if cacty is
instructed by configuration to use the net-snmp binary instead.

Change default configuration to use net-snmp by default,
but kept php-snmp dependency to not break existing installs.

Do some extra cleanup and to comply with portlint and portfmt expectations.