[mapplauncherd] Improve OOM score handling

1) The --daemon-mode parameter is renamed to --keep-oom-score (still -o), and its behavior is clarified in the documentation. If provided, oom_score_adj will not be changed. Otherwise, it will be reset to 0 for the boosted process.

2) oom_score_adj is used instead of oom_score

Unfortunately, there is a problem: We seem to be unable to touch oom_score_adj (Permission denied) for processes launched by systemd and their children. A quick search indicates this might be because systemd has CAP_SYS_RESOURCE and sets the score first. This is true even when only trying to increase the score (and thus, increase the chances of being killed).