We currently convert to and from the modem's set charset and always pass
'15' as the data coding scheme. Passing the correct data coding scheme
as third argument to CUSD only upsets the network. This contradicts 3GPP
TS 23.038. Other tools like gsm-ussd handle it the same way.
Network responses that require further actions are not yet implemented.
(some fixes and cleanups by Dan Williams)
