The following changes are included:
1) install user unit files from upstream dbus
2) use absolute paths to config for --system and --session instances
3) make socket activation of user units configurable
There has been a number of PRs to address this, so this one does the
bare minimum, which is to make the functionality available and
configurable but defaults to off.
Related PRs:
- #18382
- #18222
f7215c9b5b