Bug 933654 - 2/3: allow extra unsolicited responses.

For AT command handlers, returning NULL was a convenient alias for
asking amodem_send() to reply "OK" for them.  To allow extra
unsolicited responses after the original reply, there must be some
way to notify amodem_send() that the reply has been sent and don't
bother appending another "OK".  There are also cases that return an
empty string ("", not NULL).  The NULL value then becomes the best
choice in both literal and semantic way.
Bug 788928 - Part 2/5: support SMS variable message reference & console command
Bug 788928 - Part 3/5: add sms cmse/drpt console commands
Bug 788928 - Part 4/5: send delivery status report
Bug 788928 - Part 5/5: send CMS ERROR