vty: Make TCP port configurable and introduce telnet_init_default

Extend the vty_bind_cmd VTY command to allow to optionally specify
a port in addition to the IPv4 address.

Introduce telnet_init_default to relieve client code from having
to query the bind IPv4 address (and now the TCP port). Instead a
client only needs to pass the default TCP port to use.

Client code should use it like:

	int rc = telnet_init_default(ctx, priv, OSMO_VTY_PORT_SGSN);

Change-Id: Id5fb2faaf4311bd7284ee870526a6f87b7e260f3
diff --git a/src/vty/telnet_interface.c b/src/vty/telnet_interface.c
index 47c45fc..e090e17 100644
--- a/src/vty/telnet_interface.c
+++ b/src/vty/telnet_interface.c
@@ -105,6 +105,18 @@
 	return 0;
 }
 
+/*! Initializes telnet based VTY interface using the configured bind addr/port.
+ *  \param[in] tall_ctx \ref talloc context
+ *  \param[in] priv private data to be passed to callback
+ *  \param[in] default_port TCP port number to bind to if not explicitely configured
+ */
+int telnet_init_default(void *tall_ctx, void *priv, int default_port)
+{
+	return telnet_init_dynif(tall_ctx, priv, vty_get_bind_addr(),
+				 vty_get_bind_port(default_port));
+}
+
+
 extern struct host host;
 
 /*! close a telnet connection */