zabbix_agentd: Disable passive checks by default on new installations.

zabbix_agentd: Disable passive checks by default on new installations.

Zabbix Agent by default normally forks 10 instances to listen for incoming (passive) checks.
I, however, recommend only using active checks on an IPFire instance, so that the agent on the instance will only actively contact the Zabbix server to request a list of checks to perform instead of waiting for the server to contact the agent for every check.
This frees up some resources valuable to smaller systems and makes the agent not to listen on any TCP port, which is a possible attack surface less.

Users with an existing installation will have to manually add the parameter to their config. This will be documented in the wiki.

Signed-off-by: Robin Roevens <robin.roevens@disroot.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>