Forbid partial VTY configurations of ASPs with name asp-clnt-*

A recent commit (83db938859428ce3835239e2692d93c557b750e1) changed the
behavior of default "sctp-role" and "role" values of ASPs named
asp-clnt-* which are used by osmo_sccp_simple_client_on_ss7_id(), in
order to avoid having different default values when than function is
used, which is totally confusing for end users.

As it was already informed when submitting the mentioned commit, it
changes the behavior on that specific case, and that made some users
start having problems without a proper "your config is wrong!" error.

This commit addresses it by basically forbiding that exact use case,
that is, partially defining an asp-clnt-* ASP through VTY without
explicitly configuring a "role" and "sctp-role". With this patch,
function osmo_sccp_simple_client_on_ss7_id() will print a proper error
informing the user and will return NULL, potentially triggering an early
program exit which can then easily be fixed by using proper
configuration.

Related: SYS#6486
Change-Id: I65b5fad2ec06a9d9c521f1e3ce8aab633da95a47
---
M TODO-RELEASE
M include/osmocom/sigtran/osmo_ss7.h
M src/osmo_ss7_vty.c
M src/sccp_user.c
4 files changed, 67 insertions(+), 14 deletions(-)