Clean up in a few spots.

In chplsys, for Linux and Cygwin define chpl_getNumCoresOnThisNode() in
terms of the number of PUs returned by chpl_getNumPUsOnThisNode().

In flat/LocaleModel.chpl initialize numCores with the value returned by
chpl_getNumPUsOnThisNode(), which returns what used to be returned by
chpl_numCoresOnThisLocale().  Thus the value of numCores will be the
same as it was before all this work.
Add maxTaskPar support in qthreads tasking and numa locale model.

This adds chpl_task_getMaxPar() in qthreads tasking, and updates the
numa locale model to include a maxTaskPar member and initialization of
that.

While here, I also corrected the return types of the runtime functions
that return #PUs and maxTaskPar, in the extern decls in the flat locale
model.