`init_script_path` option can be used in `[DEFAULTS]` to run a script at the end of
backend initialisation. A new `init_pre_script` method is used to wait for backend to
reach this stage (designed to be used mostly by CLI frontend), then the usual `ready_get`
method is finished once the script is finished.
it may be necessary to run some async cleaning method (e.g. if bridge is needed), this new
method handles async cleaning method, and will be called when `SystemExit` is used, or if
`a_quit` is called explicitely.
rel 426
