currently there is no way of knowing whether a zabbix daemon will like the config file without trying to start it with one.
syntax verification/check would help with that. it would not attempt to start anything about the agent, only parse the logfile, report any potential problems and exit.

exitcode 0 should be used if all is ok, something else in the case of a failure. different exitcodes could signal about different problems (missing required param, unknown param, duplicate userparam...)

should be documented in manpages, manual, whatsnew