Fixed divide-by-zero error in MCE->yield.
Refactored code for the interval option by moving the code to the manager
process. This allows the manager process to accomodate the next available
worker and ready to run. Previoulsy, a worker taking a long time resulted
in empty time slots. Thank you, Philippe Baumgart for your patience.Revised the description on posix_exit in MCE::Core.pod.