Previously we would rely on the exit

This fixes a few nasty bugs:

 * System.Process.waitForProcess failed to keep the ProcessHandle's MVar
   alive, potentially resulting in the finalizer being run while
   waitForJobCompletion is executing. This would cause the process
   handles to be closed, resulting in waitForJobCompletion to fail.

 * waitForProcess failed to explicitly close the job, process, and IOCP
   handles.

 * waitForProcess failed to stop delegation of Ctrl-C in processes using
   jobs.