Sender/AsyncSender: Support Include directives in agent config files

[rfdparker]

When a Sender/AsyncSender object is initialised with use_config=True and config_path as the path to a zabbix_agentd/zabbix_agent2 config file, any Include directives in the config file should be respected.

This would be consistent with the behaviour of zabbix_sender, zabbix_agentd and zabbix_agent2.