gbproxy: Move the VTY code into the vty file and create public API

Create public accessors to the core of the peer to allow to
simplify the test and separate concerns.

Done with Jacob.
diff --git a/openbsc/src/gprs/gb_proxy_main.c b/openbsc/src/gprs/gb_proxy_main.c
index 1f140d4..773c983 100644
--- a/openbsc/src/gprs/gb_proxy_main.c
+++ b/openbsc/src/gprs/gb_proxy_main.c
@@ -259,6 +259,7 @@
 		LOGP(DGPRS, LOGL_ERROR, "Unable to instantiate NS\n");
 		exit(1);
 	}
+	gbproxy_init_config(&gbcfg);
 	gbcfg.nsi = bssgp_nsi;
 	gprs_ns_vty_init(bssgp_nsi);
 	gprs_ns_set_log_ss(DNS);