Allow concatenated UTF-16 SMS

Change to properly handle split surrogate pairs in concatenated SMSes, as this can happen now that we use UTF-16 instead of UCS-2 for decoding SMS messages with UCS-2 coding specified in the DCS.

To test this, a message that needs more than 2 SMS, and where one code point that needs 4 bytes is split between the two messages must be used. An example would be:

send-sms \<number\> "😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱😱" 0

(note that we can send to the same number that we are sending from).