rilmodem: Support for UCS-2 in USSD

Support to send and receive USSD strings that cannot be codified using
the GSM alphabet.
src: Support to send UCS-2 codified USSD strings

Core ofono always codified USSD strings using plain GSM alphabet.
This change uses UCS-2 in case GSM alphabet is not enough.
Merge pull request #83 from alfonsosanchezbeato/ussd-ucs2

USSD - UCS-2
src: Use EF_OPL to check roaming status

Some operators (MVNOs, or Orange/T_Mobile UK case) store in EF_OPL
their home networks, so we remove the roaming flag if the network we
are registered in is present in the file. Although this is not strictly
conformant with the standard, seems to be common practive among
operators, as no operator would care about changing the displayed name
for a network which is not among its home networks (EF_OPL stores the
index for the EF_PNN entry for a PLMN).