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.
Rephrasing my previous comment: we need the address of an lvalue for ofono_dbus_dict_append_array(), and &rats_strs is an rvalue ( the reason for this second comment is because I am not going to miss the opportunity of using rvalue and lvalue for my first time in a technical discussion :-p )