Raise or remove ceiling on max_children

In osrf_prefork.c, ABS_MAX_CHILDREN sets a hard-coded limit of 256 on max children for any C service. If you attempt to set max_children to a higher value, you'll get an error when you try to start the service.

I ran into this limit with Evergreen when trying to increase open-ils.pcrud max_children above 256 in order to accommodate some workflows in the web client that can produce hundreds of near-simultaneous requests. We can make Evergreen stop doing that, but I see no obvious reason why OpenSRF needs to enforce a hard limit here, so I'd suggest removing the limit (or at least setting it much higher).