[ssudeviceinfo] Get IMEI directly from ofono via D-Bus (+fallback)

This avoids a circular dependency between QtSystems and ssu.

Fallback to /etc/machine-id and some other files if IMEI cannot be
retrieved from ofono, and normalize just like previously.
Merge pull request #32 from thp/ofono-dbus-imei

[ssudeviceinfo] Get IMEI directly from ofono via D-Bus