[GPRS] NS: Start to use rate_ctr_group code from libosmocore

Every NS-VC now has a set of counters for incoming and outgoing
number of packets and bytes.

We also split the VTY part of the gprs_ns.c implementation into gprs_ns_vty.c
to make sure the protocol can actually be used without the VTY code being
present.
diff --git a/openbsc/src/Makefile.am b/openbsc/src/Makefile.am
index 9dc0257..177c410 100644
--- a/openbsc/src/Makefile.am
+++ b/openbsc/src/Makefile.am
@@ -27,7 +27,7 @@
 		handover_logic.c handover_decision.c meas_rep.c
 
 libvty_a_SOURCES = vty/buffer.c vty/command.c vty/vector.c vty/vty.c \
-		telnet_interface.c vty_interface_cmds.c
+		telnet_interface.c vty_interface_cmds.c vty/utils.c
 
 libsccp_a_SOURCES = sccp/sccp.c