VTY: decouple telnet_interface from 'struct gsmnet'

We want the VTY and telnet code to be independent from the BSC
application(s).  As a side note, we also like to eliminate static
global variables for 'struct gsm_network' all over the code.

As such, telnet_init() is now passed along a "private" pointer,
which getst stored in telnet_connection.priv.  This telnet_connection
is then stored in vty->priv, which in turn gets dereferenced if
anyone needs a reference to 'struct gsm_network' from the BSC vty
code.

Also:
 * vty_init() now calls cmd_init()
 * the ugliness that telnet_init() calls back into the application by means of
   bsc_vty_init() function has been removed.
 * telnet_init() now returns any errors, so the main program can exit
   e.g. if the port is already in use.
diff --git a/openbsc/include/vty/vty.h b/openbsc/include/vty/vty.h
index 23430d5..f82ada1 100644
--- a/openbsc/include/vty/vty.h
+++ b/openbsc/include/vty/vty.h
@@ -129,7 +129,7 @@
 
 /* Prototypes. */
 void vty_init(const char *name, const char *version, const char *copyright);
-int vty_read_config_file(const char *file_name);
+int vty_read_config_file(const char *file_name, void *priv);
 void vty_init_vtysh (void);
 void vty_reset (void);
 struct vty *vty_new (void);