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_uxst.c b/src/proto_uxst.c
index 3ea468f..5f56ad5 100644
--- a/src/proto_uxst.c
+++ b/src/proto_uxst.c
@@ -493,7 +493,7 @@
  * 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 = { "UNIX", { }, {
 	{ "gid",   bind_parse_gid,   1 },      /* set the socket's gid */
 	{ "group", bind_parse_group, 1 },      /* set the socket's gid from the group name */
 	{ "mode",  bind_parse_mode,  1 },      /* set the socket's mode (eg: 0644)*/