- Improved handling of extension-number string (as per review)
- Guard against a buffer-overflow if mobile sends a too-long USSD
- declare some function-parameters const
- fix gsm_ts_name function to display the right BTS number (bts->nr rather than bts->bts_nr)
This patch removes the need of static global variables and introduces a new,
caller-allocated 'struct ussd_request' that needs to be passed to the various
functions.