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/.gitignore b/openbsc/.gitignore
index ca73db6..fc3d0bf 100644
--- a/openbsc/.gitignore
+++ b/openbsc/.gitignore
@@ -53,6 +53,7 @@
 src/nat/bsc_nat
 src/gprs/osmo-sgsn
 src/gprs/osmo-gbproxy
+src/gprs/osmo-gtphub
 src/osmo-bsc_nat/osmo-bsc_nat
 
 #tests
@@ -78,6 +79,7 @@
 tests/sgsn/sgsn_test
 tests/subscr/subscr_test
 tests/oap/oap_test
+tests/gtphub/gtphub_test
 
 tests/atconfig
 tests/atlocal