Changes

library/gsup: rework f_gen_{tr,ts}_ss_ies() and SS templates

* Add missing param restrictions to SS related templates.
* f_gen_{tr,ts}_ss_ies(): proper template param restrictions.
* f_gen_ts_ss_ies(): return a template, not a value.
* f_gen_ts_ss_ies(): append IEs using list index.
* f_gen_ts_ss_ies(): isvalue() -> istemplatekind()
                     to allow passing template lists.
* f_gen_tr_ss_ies(): istemplatekind() -> ispresent()
                     to allow passing template lists.
* f_gen_tr_ss_ies(): 'omit' -> '*' for optional params.

This patch makes the following testcases pass [again]:

* TC_mt_ussd_for_unknown_subscr,
* TC_proc_ss_for_unknown_session.

At some point osmo-msc started including the Source Name IE in
OSMO_GSUP_MSGT_PROC_SS_ERROR messages, which we didn't expect.
We still do not expect that IE, but now we put '*' in place of
the Destination Name IE by default, which matches it.

A follow-up commit will add support fot the Destination Name IE.

Change-Id: Ia611ffa3845d48c5553c70f10981aa029d00d873