BUG/MINOR: mworker: wrong signals during startup

BUG/MINOR: mworker: wrong signals during startup

Since the new master-worker model in 3.1, signals are registered in
step_init_3(). However, those signals were supposed to be registered
only for the worker or the standalone mode. It would call the wrong
callback in the master even during configuration parsing.

The patch set the signals handler to NULL for the master so it does
nothing until they really are registered.

Must be backported as far as 3.1.