libvlr/vlr.c: forward unhandled GSUP messages towards MSC

Some internal sub-systems, such as SS/USSD or SMS implementation,
may also need to use GSUP connection with HLR. Previously, it was
only available within the libvlr code, and nowhere else.

Let's introduce the generic GSUP message router, which will
receive messages unhandled by VLR itself, and route them to
a handler depending on the message type.

Change-Id: Ib8146ce5788c8f249dcaa39d61bd0388574bf892
3 files changed