MINOR: listener/config: make "thread" always support up to LONGBITS

The difference is subtle but in one place there was MAXTHREADS and this
will not work anymore once it goes over 64.
diff --git a/src/listener.c b/src/listener.c
index 59c31cf..6b08802 100644
--- a/src/listener.c
+++ b/src/listener.c
@@ -1808,8 +1808,8 @@
 		memprintf(err, "'%s' multiple thread-groups are not supported", args[cur_arg + 1]);
 		return ERR_ALERT | ERR_FATAL;
 	}
-	
-	if (parse_process_number(sep, &thread, MAX_THREADS, NULL, err)) {
+
+	if (parse_process_number(sep, &thread, LONGBITS, NULL, err)) {
 		memprintf(err, "'%s' : %s", sep, *err);
 		return ERR_ALERT | ERR_FATAL;
 	}