Restore code to handle '-fmax-worker-args' flag

When i had a pass through DynFlags I've noticed dead code:

https://phabricator.haskell.org/D1727

maxWorkerArgs handling was accidentally lost 3 years ago in major update changeset:0831a12e

The consensus is to try to put option handling back.

Edited Mar 10, 2019 by Sergei Trofimovich

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