[iserv] move forkIO
[iserv] move forkIO

This moves the forkIO into the `startSlave` function from the
`startSlave'` function, such that this allows consumers to call
`forkSlave'` if they want the blocking behaviour.

Reviewers: bgamari, austin

Reviewed By: bgamari

Subscribers: Ericson2314, ryantrinkle, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3601

ezyang/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc