MINOR: listener: add a scope field in the bind keyword lists

This scope is used to report what the keywords are used for (eg: TCP,
UNIX, ...). It is now reported by bind_dump_kws().
diff --git a/src/proto_tcp.c b/src/proto_tcp.c
index fe89569..6b1b15e 100644
--- a/src/proto_tcp.c
+++ b/src/proto_tcp.c
@@ -1842,7 +1842,7 @@
  * the config parser can report an appropriate error when a known keyword was
  * not enabled.
  */
-static struct bind_kw_list bind_kws = {{ },{
+static struct bind_kw_list bind_kws = { "TCP", { }, {
 #ifdef TCP_DEFER_ACCEPT
 	{ "defer-accept",  bind_parse_defer_accept, 0 }, /* wait for some data for 1 second max before doing accept */
 #endif