[FIX] LegacyMultiProc hangs up indefenitely

This PR relates to nipy#2700, and should fix the problem
underlying nipy#2548.

I first considered adding a control thread that monitors
the `Pool` of workers, but that would require a large overhead
keeping track of PIDs and polling very often.

Just adding the core file of [bpo-22393](python/cpython#10441)
should fix nipy#2548

Author: oesteban