Adjust chpl_task_maxPar() to take maxThreads into account.

For fifo and massivethreads tasking, the chpl_task_getMaxPar() function
wasn't taking into account any limitation on the number of threads.  Now
it does.
Simplify chpl_task_getMaxPar() in response to review.
Merge pull request #227 from gbtitus/maxTaskPar-reflect-numThreads

Adjust chpl_task_maxPar() to take maxThreads into account.

For fifo and massivethreads tasking, the chpl_task_getMaxPar() function
wasn't taking into account any limitation on the number of threads. Now
it does.

[Reviewed by @ronawho.]