Correct vzps usage

This PR fixes what was introduced in https://github.com/cfengine/core/pull/956
Indeed, the classes were correctly defined, but I made a mistake in the implementation, and the vzps command was not used.

Here it is fixed by introducing the VPSHARDCLASS PlatformContext, used for the PS commands and parameters.
By default, its value is the same as VSYSTEMHARDCLASS, but on *vz system with vzps, it gets a new value

The benefit of this new variable is that we can use as many PS command as we need on different system (and not only copying the one from linux), without the risk of changing VSYSTEMHARDCLASS, which as a lot lot lot lot of impacts everywhere in the code