Sets up a child setup function.
The child setup function will be called after
fork() but before
exec() on the child’s side.
destroy_notify will not be automatically called on the child’s side
of the fork(). It will only be called when the last reference on the
GSubprocessLauncher is dropped or when a new child setup function is given.
NULL can be given as
child_setup to disable the functionality.
Child setup functions are only available on UNIX.
Available since: 2.40
This method is not directly available to language bindings.