This changeset factors out gsm_transaction as something independent
of call control in preparation to re-use the code from SMS. A
transaction is uniquely identified by either its callref, or by
a tuple of (transaction_id, protocol, subscriber).
This is Harald's reworked MNCC base, slowly heading towards integration
into master. The key changes are:
* provide much more structure to the data in gsm_mncc
* encode_* and decode_* functions now take a structure rather than tons
of individual arguments (whose order nobody can remember)
* make sure we don't have copies of the same code everywhere by introducing
mncc_set_cause() and mncc_release_ind()
* save horizontal screen space if possible
* make sure we break lines > 80 characters
Harald Welte