enable telnet VTY bind address config for various programs

Following the 'line vty'/'bind A.B.C.D' command added in libosmocore, use the
configured address to set the telnet bind for the VTY line. It is now possible
to publish the VTY on a specific local interface (including 0.0.0.0 aka "any").

Implement in all of:
  osmo-gbproxy
  osmo-gtphub
  osmo-sgsn
  osmo-bsc
  osmo-bsc_nat
  osmo-bsc_mgcp
  osmo-nitb

In some of these main programs, move the telnet initialization below the
configuration parsing.

Historically, this was not a good idea for programs using bsc_init.c (aka
bsc_bootstrap_network()), since they expected a gsm_network struct pointer in
((struct telnet_connection*)vty->priv)->priv, so that telnet had to be either
initialized or replaced by a dummy struct. In the meantime, the gsm_network
struct is not actually looked up in a priv pointer but in the static bsc_vty.c
scope (bsc_gsmnet), so this limitation is mere legacy (even though said legacy
is still there in an "#if 0" chunk).

In the other binaries I have briefly looked at the init sequence dependencies
and found no reason to initialize telnet above the config file parsing. In any
case, I have tested every single one of abovementioned binaries to verify that
they still parse the example config successfully and launch, allowing VTY
connections on the configured address(es). I hope this suffices.

In all of the above, log VTY address and port. LOGL_INFO is disabled by default
in some of the logging scopes, and since it is a single log message right at
program launch, I decided for the slightly more aggressive LOGL_NOTICE.
diff --git a/openbsc/src/gprs/gb_proxy_main.c b/openbsc/src/gprs/gb_proxy_main.c
index f82fb5b..0c3cfbe 100644
--- a/openbsc/src/gprs/gb_proxy_main.c
+++ b/openbsc/src/gprs/gb_proxy_main.c
@@ -252,10 +252,6 @@
 	rate_ctr_init(tall_bsc_ctx);
 	osmo_stats_init(tall_bsc_ctx);
 
-	rc = telnet_init(tall_bsc_ctx, &dummy_network, OSMO_VTY_PORT_GBPROXY);
-	if (rc < 0)
-		exit(1);
-
 	bssgp_nsi = gprs_ns_instantiate(&proxy_ns_cb, tall_bsc_ctx);
 	if (!bssgp_nsi) {
 		LOGP(DGPRS, LOGL_ERROR, "Unable to instantiate NS\n");
@@ -274,6 +270,14 @@
 		exit(2);
 	}
 
+	/* start telnet after reading config for vty_get_bind_addr() */
+	LOGP(DGPRS, LOGL_NOTICE, "VTY at %s %d\n",
+	     vty_get_bind_addr(), OSMO_VTY_PORT_GBPROXY);
+	rc = telnet_init_dynif(tall_bsc_ctx, &dummy_network,
+			       vty_get_bind_addr(), OSMO_VTY_PORT_GBPROXY);
+	if (rc < 0)
+		exit(1);
+
 	if (!gprs_nsvc_by_nsei(gbcfg.nsi, gbcfg.nsip_sgsn_nsei)) {
 		LOGP(DGPRS, LOGL_FATAL, "You cannot proxy to NSEI %u "
 			"without creating that NSEI before\n",