FCPPluginServerMessage: Fix unused constant PARAM_PREFIX and use it

- Similar to FCPPluginClientMessage, this constant should be the name of
the sub-SimpleFieldSet into which the params are put. But instead, it
was not being used to name the subset, and also had the wrong value.
- Eclipse says this was not being used anywhere in fred. Also, this
shouldn't have been used by client plugin code IMHO. So it should be
safe to change it
- Will be needed for unit tests.