dbus: add socket activation but do not enable it

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

(cherry picked from commit f7215c9b5b47dfb0a6dbe87ff33d7730729a32e5)
Signed-off-by: Domen Kožar <domen@dev.si>