Create extra configuration option for zabbix_sender

Right now, zabbix_sender is coupled with Zabbix agent 1. That means, the zabbix_sender binary is only build if --enable-agent is passed to the configure script. This also means that I have to set USE="agent" on my Gentoo machines, even though I just want to use zabbix_sender with agent 2. Please consider further decoupling zabbix_sender from agent 1, so that it can be enabled independently.