MINOR: listener: move bind_proc and bind_thread to struct settings

As mentioned previously, these two fields come under the settings
struct since they'll be used to bind receivers as well.
diff --git a/src/proto_udp.c b/src/proto_udp.c
index be100b5..5ec8bc7 100644
--- a/src/proto_udp.c
+++ b/src/proto_udp.c
@@ -279,7 +279,7 @@
 
 	if (listener->bind_conf->frontend->mode == PR_MODE_SYSLOG)
 		fd_insert(fd, listener, syslog_fd_handler,
-		          thread_mask(listener->bind_conf->bind_thread) & all_threads_mask);
+		          thread_mask(listener->bind_conf->settings.bind_thread) & all_threads_mask);
 	else {
 		err |= ERR_FATAL | ERR_ALERT;
 		msg = "UDP is not yet supported on this proxy mode";