ka-customizer: use GSubprocess instead of spawn_awync
ka-customizer: use GSubprocess instead of spawn_awync

It will allow for nicer output handling
https://phabricator.endlessm.com/T13878

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib