sgsn/ctrl: Add ctrl interface, implement listing subscribers

Add the control interface with no hierachy right now and implement
the first command to list IMSI + Context Address of active sessions.
sgsn_cmd_handle could share more code with bsc variant.

Fixes: SYS#264, SYS#265
diff --git a/openbsc/src/gprs/Makefile.am b/openbsc/src/gprs/Makefile.am
index f4b1354..90c376c 100644
--- a/openbsc/src/gprs/Makefile.am
+++ b/openbsc/src/gprs/Makefile.am
@@ -19,6 +19,9 @@
 
 osmo_sgsn_SOURCES =	gprs_gmm.c gprs_sgsn.c gprs_sndcp.c gprs_sndcp_vty.c \
 			sgsn_main.c sgsn_vty.c sgsn_libgtp.c \
-			gprs_llc.c gprs_llc_vty.c crc24.c
-osmo_sgsn_LDADD = 	$(top_builddir)/src/libcommon/libcommon.a \
-			-lgtp $(OSMO_LIBS)
+			gprs_llc.c gprs_llc_vty.c crc24.c \
+			sgsn_ctrl.c
+osmo_sgsn_LDADD = 	\
+			$(top_builddir)/src/libctrl/libctrl.a \
+			$(top_builddir)/src/libcommon/libcommon.a \
+			-lgtp $(OSMO_LIBS) $(LIBOSMOABIS_LIBS)