Have spawnLocal inherit the masking state of the parent

[facundominguez]

This will have an effect in callLocal action which will run action in whatever the current masking state is. It is also required for correctness of callLocal in PR #180, I believe (races there otherwise).

Also, may or may not require submitting a fix to:
https://ghc.haskell.org/trac/ghc/ticket/10149

Here's an attempt to address it without fixing base:
https://github.com/haskell-distributed/distributed-process/compare/wip/inherit-masking-state
needs testing.