library/GSUP_Types.ttcn: refactor PROC_SS IEs generation

Both ts_GSUP_PROC_SS_ERR() and tr_GSUP_PROC_SS_ERR() templates
used to compose the set of GSUP IEs manually, while similar ones
were using both f_gen_ts_ss_ies() and f_gen_tr_ss_ies().

This led to the following problems:

  - tr_GSUP_PROC_SS_ERR was not tolerant to omitted
    message class IE, which was recently introduced;
  - code duplication.

Let's modify the both functions in order to accept an optional
Cause IE value, which is omitted by default, and use them in
the both templates.

Change-Id: I5cd6d2bc754bcedd1e721b3bd95ada9cdd44bcf0
1 file changed