Incremental output from Fork

[henrikbjorn]

I use forks for processing jobs of a queue and it would be a good addition to get feedback incrementally instead of a big lump of output in the end.

[kriswallsmith]

We can try doing this using progress() and notify(). I've started work here: 8e2a7c3. If a child process writes to the FIFO I think it will need to signal the parent process to collect that message. The parent process will need to listen using ->addSignalListener().