gb: Add functions to access the LL part of the NS-VC objects

Adds the functions gprs_ns_ll_copy() and gprs_ns_ll_clear(). Renames
gprs_ns_format_peer() to gprs_ns_ll_str(). All of these functions
uniformly access the link layer part within the NS-VC objects.

Sponsored-by: On-Waves ehf
diff --git a/src/gb/libosmogb.map b/src/gb/libosmogb.map
index 0270db8..a21a7ac 100644
--- a/src/gb/libosmogb.map
+++ b/src/gb/libosmogb.map
@@ -54,7 +54,9 @@
 gprs_ns_tx_status;
 gprs_ns_tx_unblock;
 gprs_ns_vty_init;
-gprs_ns_format_peer;
+gprs_ns_ll_str;
+gprs_ns_ll_copy;
+gprs_ns_ll_clear;
 
 gprs_nsvc_create;
 gprs_nsvc_delete;