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/src/gprs/Makefile.am b/openbsc/src/gprs/Makefile.am
index c8e3696..0dd38f9 100644
--- a/openbsc/src/gprs/Makefile.am
+++ b/openbsc/src/gprs/Makefile.am
@@ -11,6 +11,7 @@
 bin_PROGRAMS = osmo-gbproxy
 
 if HAVE_LIBGTP
+bin_PROGRAMS += osmo-gtphub
 if HAVE_LIBCARES
 bin_PROGRAMS += osmo-sgsn
 endif
@@ -33,3 +34,8 @@
 			$(top_builddir)/src/libcommon/libcommon.a \
 			-lgtp $(OSMO_LIBS) $(LIBOSMOABIS_LIBS) $(LIBCARES_LIBS) \
 			$(LIBCRYPTO_LIBS) -lrt
+
+osmo_gtphub_SOURCES =	gtphub_main.c gtphub.c gtphub_ext.c gtphub_vty.c
+osmo_gtphub_LDADD = 	\
+			$(top_builddir)/src/libcommon/libcommon.a \
+			-lgtp $(LIBOSMOCORE_LIBS) $(LIBOSMOVTY_LIBS) -lrt