repurpose the child_functions array to be child_operations instead.

Instead of doing different things per child, we want to be able to have
a child do things other than just random syscalls.

This involves turning the child_random_syscalls() & child_process() functions
inside out, but should allow us to do a lot more interesting things in
children/ now.

For now the child_read_all_files() child is disabled (as it was previously).

I might reintroduce multiple child types later, but right now things in children/
should be 'one shot' and not involve lengthy loops, because we're going to be
moving the "we are making progress" indicators up into child_process too (See FIXME).
we don't care about child_process's return value.