When 'zabbix_agentd' is started with '-p' option it shows all known items.
Problem arises when a user puts under UserParameter a script with side-effects (e.g. rsync). User can reasonably assume that '-p' is harmless to use, kind of 'preview' but instead an incident may be caused.
- With '-p' option just show the names of UserParameter-s but do not evaluate them.
- With '-t' option evaluate the UserParameter.