Changes for https://fedoraproject.org/wiki/Changes/EnableSystemdOomd.
Backports primarily PR #18361, #18444, and #18401 (#18401 is not merged
at the time of writing this commit) + some minor PRs to handle conflicts.
Creates systemd-oomd-defaults subpackage to install unit drop-ins that
will configure systemd-oomd to monitor and act.
