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
diff --git a/doc/examples/osmo-hlr.cfg b/doc/examples/osmo-hlr.cfg
index ebc22ec..35d942d 100644
--- a/doc/examples/osmo-hlr.cfg
+++ b/doc/examples/osmo-hlr.cfg
@@ -21,3 +21,4 @@
 hlr
  gsup
   bind ip 127.0.0.1
+ ussd route prefix *#100# internal own-msisdn