Process creation on Windows is needlessly serialised #381

When close_fds = True, we can explicitly pass the processes that the child is allowed to inherit. This completely avoids the problem of GHC bug #2650 that made locking necessary.

In a microbenchmark, spawning 200 processes with close_fds = True concurrently takes about 1.5s on my machine, with over 40% of the time (0.7s) wasted due to waiting on the lock.

While working on speeding up GHC's downsweep pass, I found that on Windows a lot of time was spent on process spawning (e.g. to run the C preprocessor), and that this serialisation was a significant component of that. In a rebuild of the vulkan package, the downsweep pass spawns a huge amount of subprocesses, and removing this locking takes the time spent in downsweep from 31s to 27s (4s total timesave).