MINOR: log: Save alerts and warnings emitted during HAProxy startup

MINOR: log: Save alerts and warnings emitted during HAProxy startup

Because we can't always display the standard error messages when HAProxy is
started, all alerts and warnings emitted during the startup will now be saved in
a buffer. It can also be handy to store these messages just in case you
missed something during the startup

To implement this feature, Alert and Warning functions now relies on
display_message. The difference is just on conditions to call this function and
it remains unchanged. In display_message, if MODE_STARTING flag is set, we save
the message.