MINOR: hlua: Display debug messages on stderr only in debug mode

Debug Messages emitted in lua using core.Debug() or core.log() are now only
displayed on stderr if HAProxy is started in debug mode (-d parameter on the
command line). There is no change for other message levels.

This patch should fix the issue #879. It may be backported to all stable
versions.
diff --git a/src/hlua.c b/src/hlua.c
index 390191e..e7cac65 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -945,6 +945,9 @@
 
 	send_log(px, level, "%s\n", trash.area);
 	if (!(global.mode & MODE_QUIET) || (global.mode & (MODE_VERBOSE | MODE_STARTING))) {
+		if (level == LOG_DEBUG && !(global.mode & MODE_DEBUG))
+			return;
+
 		get_localtime(date.tv_sec, &tm);
 		fprintf(stderr, "[%s] %03d/%02d%02d%02d (%d) : %s\n",
 		        log_levels[level], tm.tm_yday, tm.tm_hour, tm.tm_min, tm.tm_sec,