The quasi-fork child was waiting for an explicit signal from the parent
before terminating. This synchronization step is unnecessary, and
it makes the child hang if the parent terminates for any reason
while the fork operation is in progress.
In that scenario, and consistently with the Posix version, the child
now completes the fork operation and terminates freely when done.

This fixes https://github.com/MSOpenTech/redis/issues/228