[GPRS] Properly connect GPRS SM with LIBGTP for PDP context activation

* store LLC SAPI as part of PDP ctx
* store NSEI + BVCI as part of MM ctx
* export gsm48_tx_gsm_act_pdp_acc() and call it from sgsn_libgtp.c
* create and use gsm48_tx_gsm_act_pdp_rej for error cases
* print SAPI as part of VTY show pdp
diff --git a/openbsc/src/gprs/sgsn_vty.c b/openbsc/src/gprs/sgsn_vty.c
index b59529c..5584824 100644
--- a/openbsc/src/gprs/sgsn_vty.c
+++ b/openbsc/src/gprs/sgsn_vty.c
@@ -168,7 +168,7 @@
 			 struct sgsn_pdp_ctx *pdp)
 {
 	vty_out(vty, "%sPDP Context IMSI: %s, SAPI: %u, NSAPI: %u%s",
-		pfx, pdp->mm->imsi, 2342 /* FIXME */, pdp->nsapi, VTY_NEWLINE);
+		pfx, pdp->mm->imsi, pdp->sapi, pdp->nsapi, VTY_NEWLINE);
 	vty_out(vty, "%s  APN: %s\n", pfx, pdp->lib->apn_use.v);
 	/* FIXME: statistics */
 }