mgcp_verify_ci(): return meaningful error codes

Instead of just -1, return RFC3435 error codes that can be used to compose a
FAIL message response. Note that the return value stays compatible in that it
returns 0 on a valid Connection Identifier, nonzero otherwise.

The idea is to be able to distinguish between "Conn ID not found" and "Conn ID
invalid" in mgcp_test.c's expected output, in upcoming change
I8d6cc96be252bb486e94f343a8c7cae641ff9429.

Change-Id: Ifc17f2893cc4b9a865f3ffcb9888bbf1039337a6
2 files changed