BUG/MINOR: Allocate the log buffers before the proxies startup

Since the commit cd7879adc ("BUG/MEDIUM: threads: Run the poll loop on the main
thread too"), the log buffers are allocated after the proxies startup. So log
messages produced during this startup was ignored.

To fix the bug, we restore the initialization of these buffers before proxies
startup.

This is specific to threads, no backport is needed.

src/haproxy.c

@@ -1796,6 +1796,11 @@ static void init(int argc, char **argv)
 		exit(1);
 	}
 
+	if (!init_log_buffers()) {
+		Alert("failed to initialize log buffers.\n");
+		exit(1);
+	}
+
 	/*
 	 * Note: we could register external pollers here.
 	 * Built-in pollers have been registered before main().