core: check for SERVICE_RELOAD_NOTIFY in manager_dbus_is_running
This ensures that systemd won't erronously disconnect from the system
bus in case a bus recheck is triggered immediately after the bus service
emits `RELOADING=1`.
This fixes an issue where systemd-logind sometimes randomly stops
receiving `UnitRemoved` after a system update.
This also handles SERVICE_RELOAD_SIGNAL just in case somebody ever
creates a D-Bus broker implementation that uses `Type=notify-reload`.