BUG/MEDIUM: mworker: unregister the signals of main()

BUG/MEDIUM: mworker: unregister the signals of main()

The signal_register_fct() does not remove the handlers assigned to a
signal, but add a new handler to a list.

We accidentality inherited the handlers of the main() function in the
master process which is a problem because they act on the proxies.

The side effect was to stop the MASTER proxy which handle the master CLI
on a SIGUSR1, and to display some debug info when doing a SIGHUP and a
SIGQUIT.