Child processes have no way to use atexit functionality

[Joshua Harlow]

Currently in nova, when children are spawned via the forking model there exists no way for said child process to have its own set of atexit handlers (due to the usage of os._exit). Say each process has a local log handler and said log handler needs to be flushed before the child process exits. It would be useful to let atexit be used or an alternative mechanism be provided for children to invoke cleanup tasks before they are shutdown.