List valid modem technologies in RadioSettings interface

This branch adds ModemTechnologies property to org.ofono.RadioSettings interface. This property lists valid RATs for the modem. The changes also ensure that org.ofono.RadioSettings is available when there is no SIM, which is required to access FastDormancy (needed for power management) and ModemTechnologies (needed by system settings) properties.
That's cause I left out a "&" in my ofono_dbus_dict_append_array() call.   The last parameter should've been "&rats_strs".

That said, I think your code may be correct, although the first version may have been less cryptic.