USSD: Add support for internal USSD handlers

There are some requests that are best served inside the HLR, as it
has access to subscriber information such as MSISDN and IMSI.

This unfortunately required quite some restructuring of the USSD
related structures including the VTY syntax for adding routes.

The default config file has been updated to replicate the *#100#
built-in behavior of old OsmoNITB.

Closes: OS#2566
Change-Id: I1d09fab810a6bb9ab02904de72dbc9e8a414f9f9
7 files changed