(Merge 3.2) Issue #12310: finalize the old process after _run_after_forkers()

multiprocessing: Process._bootstrap() keeps a reference to the old process to
delay its finalization until after _run_after_forkers() as been executed. This
change should fix a crash on Mac OS X Tiger when a lock is released after a
fork.

Patch written by Charles-François Nataliv and Antoine Pitrou.