Add GTP hub initial code base.

First steps towards a new GTP hub. The aim is to mux GTP connections, so that
multiple SGSN <--> GGSN links can pass through a single point. Background:
allow having more than one SGSN, possibly in various remote locations.

The recent addition of OAP to GSUP is related to the same background idea.

(This is a collapsed patch of various changes that do not make sense to review
in chronological order anymore, since a lot of it has thorougly transmorphed
after it was first committed.)

Sponsored-by: On-Waves ehf
diff --git a/openbsc/configure.ac b/openbsc/configure.ac
index 8b7ce62..098e5b4 100644
--- a/openbsc/configure.ac
+++ b/openbsc/configure.ac
@@ -210,6 +210,7 @@
     tests/sgsn/Makefile
     tests/subscr/Makefile
     tests/oap/Makefile
+    tests/gtphub/Makefile
     doc/Makefile
     doc/examples/Makefile
     Makefile)