RTSolver: RTSteps not initialized

The counter RTSteps in RTSolver is never actually initialized in the class constructor. This can lead to RT solvers not getting the initial reference time on systems in which RTSteps is not automatically initialized to 0: for example, in RTPOSIXSolver::Wait(), the call to clock_gettime() is skipped.

The fix is trivial and already written in the branch 'rtposixsolver_fix`, which I'm about to merge.