lib, zebra: Keep zebra on-rib-process script in frr.conf

After the change:

```
$ grep on-rib-process /etc/frr/frr.conf
zebra on-rib-process script script4

$ systemctl restart frr

$ vtysh -c 'show run' | grep on-rib-process
zebra on-rib-process script script4
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>
(cherry picked from commit 1fe1f8d)

lib/frrscript.c

@@ -27,6 +27,16 @@ struct frrscript_names_head frrscript_names_hash;
 
 void _lua_decode_noop(lua_State *L, ...) {}
 
+void frrscript_names_config_write(struct vty *vty)
+{
+	struct frrscript_names_entry *lua_script_entry;
+
+	frr_each (frrscript_names, &frrscript_names_hash, lua_script_entry)
+		if (lua_script_entry->script_name[0] != '\0')
+			vty_out(vty, "zebra on-rib-process script %s\n",
+				lua_script_entry->script_name);
+}
+
 /*
  * Wrapper for frrscript_names_add
  * Use this to register hook calls when a daemon starts up

lib/frrscript.h

@@ -44,6 +44,8 @@ struct frrscript_names_entry {
 
 extern struct frrscript_names_head frrscript_names_hash;
 
+extern void frrscript_names_config_write(struct vty *vty);
+
 int frrscript_names_hash_cmp(const struct frrscript_names_entry *snhe1,
 			     const struct frrscript_names_entry *snhe2);
 uint32_t frrscript_names_hash_key(const struct frrscript_names_entry *snhe);

zebra/zebra_vty.c

@@ -3794,6 +3794,10 @@ static int config_write_protocol(struct vty *vty)
 	if (!zebra_nhg_recursive_use_backups())
 		vty_out(vty, "no zebra nexthop resolve-via-backup\n");
 
+#ifdef HAVE_SCRIPTING
+	frrscript_names_config_write(vty);
+#endif
+
 	if (rnh_get_hide_backups())
 		vty_out(vty, "ip nht hide-backup-events\n");