mainloop-io-worker: take care that the worker-threads option can set the max_threads,

therefore we should set it to default if it isn't set by command line option

Signed-off-by: Juhász Viktor <viktor.juhasz@balabit.com>
Merge pull request #148 from juhaszviktor/worker_threads

mainloop-io-worker: take care that the worker-threads option can set the...