BUG/MINOR: mworker: detach from tty when received READY from worker

BUG/MINOR: mworker: detach from tty when received READY from worker

Some master process' initialization steps are conditioned by receiving the
READY message from worker (pidfile creation, forwarding READY message to the
launching parent). So, master process can not do these initialization routines
before.

If the master process fails, while creating pid or forwarding the READY to the
parent in daemon mode, he exits with a proper alert message. In daemon mode we
no longer see such message, as process is already detached from the tty.

To fix this, as these alerts could be very useful, let's detach the master
process from the tty after his last initialization steps in _send_status.