[gprs] gb_proxy: Send proper BSSGP STATUS msg in error case

In order to reuse the existing bssgp_tx_* functions without pulling
in the dependencies of gprs_bssgp.c, we have to move those functions
to gprs_bssgp_util.c

Furthermore, we can remove gbprox_nsi and replace it with bssgp_nsi,
and we can do proper processing of BVC-RESET messages coming from
the SGSN on the signalling BVC.  In that case we need to send RESET
messages to all the BSS.
diff --git a/openbsc/src/gprs/Makefile.am b/openbsc/src/gprs/Makefile.am
index 2b42923..7cb7ba6 100644
--- a/openbsc/src/gprs/Makefile.am
+++ b/openbsc/src/gprs/Makefile.am
@@ -6,10 +6,11 @@
 noinst_LIBRARIES = libsgsn.a
 
 libsgsn_a_SOURCES = gprs_ns.c gprs_bssgp.c gprs_llc.c gsm_04_08_gprs.c \
-		crc24.c gprs_sgsn.c
+		crc24.c gprs_sgsn.c gprs_bssgp_util.c
 
 osmo_gbproxy_SOURCES = gb_proxy.c gb_proxy_main.c gb_proxy_vty.c \
-			gprs_ns.c $(top_srcdir)/src/socket.c $(top_srcdir)/src/debug.c
+			gprs_ns.c gprs_bssgp_util.c \
+			$(top_srcdir)/src/socket.c $(top_srcdir)/src/debug.c
 osmo_gbproxy_LDADD = $(top_builddir)/src/libvty.a
 
 osmo_sgsn_SOURCES = sgsn_main.c sgsn_vty.c \