Fix bug that caused ghci to create a large number of kqueues and pipes on OS X.

This is caused because ensureIOManagerIsRunning is repeatedly called and this was initializing new IO managers on each call. Fixed so that a new manager is not created if one already exists.
Merge branch 'parallel-iomanager-reimp-7' of github.com:AndreasVoellmy/ghc-base-arv into parallel-iomanager-reimp-7