systemd: start as the D-Bus user/group, rather than root

When starting as root files in /proc/self/fdinfo/ will be owned as root and set to 400, so we cannot read them. Nowadays it is not necessary to start as root when running under systemd, so just add User/Group with the configured user to the system unit.

Signed-off-by: Luca Boccassi bluca@debian.org

Split out of !398 (merged)