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

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.

(cherry picked from commit f98d821b942b4f392e77431c320aa895ca97369e)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 604a647d4d7ff310d0ed1e9f98531b6a0c9e332e)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>