zabbix_agentd: Configfile reorganization

zabbix_agentd: Configfile reorganization

- Restrict default main config to only the bare minimum options
  and add upstream provided config as example file.
- Remove /etc/zabbix_agentd from backup and instead add only
  zabbix_agentd.conf and subdirs 'scripts' and 'zabbix_agentd.d' to
  the backup.
- Move ipfire managed userparameter_pakfire.conf from
  user managed dir /etc/zabbix_agentd/zabbix_agent.d to
  ipfire managed dir /var/ipfire/zabbix_agentd/userparameters
- Add Include line to existing zabbix_agentd.conf to include
  the new ipfire managed config dir /var/ipfire/zabbix_agentd/...
- Add and include mandatory IPFire specific agent configuration
  which should never be changed by the user.

Signed-off-by: Robin Roevens <robin.roevens@disroot.org>