gb_proxy: Add ctrl interface and nsvc-state, gbproxy-state commands

This patch adds a control interface to osmo-gbproxy as well as the first
two commands to query the state of each NSVC and gbproxy peer.

The "nsvc-state" command replies with
nsei, nsvci, local state, role, remote state of all NSVCs.

The "gbproxy-state" command replies with
nsei, bvci, mcc, mnc, lac, rac, and state of each peer.

Entries are separated by a newline '\n' character. If there are no
entries an empty list is returned. This behaviour is similar to that of
the subscriber-list-active-v1 command in osmo-sgsn.

$ ./osmo_ctrl.py -d 127.0.0.1 -p 4263 -g nsvc-state
Got message: b'GET_REPLY 23 nsvc-state 101,101,DEAD,BLOCKED,SGSN,DEAD,UNBLOCKED\n'
$ ./osmo_ctrl.py -d 127.0.0.1 -p 4263 -g gbproxy-state
Got message: b'GET_REPLY 4871085901306801158 gbproxy-state '

Change-Id: I82c74fd0bfcb9ba4ec3619d9fdaa0cae201b3177
Ticket: OS#3281, SYS#4235
Sponsored-by: On-Waves ehf

include/osmocom/sgsn/gb_proxy.h

@@ -184,6 +184,9 @@ struct gbproxy_link_info {
 int gbproxy_vty_init(void);
 int gbproxy_parse_config(const char *config_file, struct gbproxy_config *cfg);
 
+/* gb_proxy_ctrl.c */
+int gb_ctrl_cmds_install(void);
+
 
 /* gb_proxy.c */
 int gbproxy_init_config(struct gbproxy_config *cfg);