A fork short of room refuses before it pauses its source

A fork on a server with no room left for memory images could give up after a fixed minute while its source's image was still being written, and leave the source unable to wake. A fork now books the room its source's wake needs before it pauses anything: a server without it answers 503 no_capacity at once, naming the room, with the source still running. And a fork waits while its source's image is still being written.