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.
Not really, ofono_dbus_dict_append() argument is the address of a pointer to the values to be appended, so you need a variable to hold that pointer (you cannot use directly rats_strs as that is directly the address to the values).

To make it clearer I made a more correct declaration:

const char *(*strs)[] = &rats_strs;

So "strs" is a pointer to an array of pointers to const char.