MINOR: threads: Don't start when device a detection module is used

For now, we don't know if device detection modules (51degrees, deviceatlas and
wurfl) are thread-safe or not. So HAproxy exits with an error when you try to
use one of them with nbthread greater than 1.

We will ask to maintainers of these modules to make them thread-safe or to give
us hints to do so.
diff --git a/src/51d.c b/src/51d.c
index 1f1a5f9..baeb2c2 100644
--- a/src/51d.c
+++ b/src/51d.c
@@ -547,6 +547,11 @@
 	if (!global_51degrees.data_file_path)
 		return 0;
 
+	if (global.nbthread > 1) {
+		Alert("51Degrees: multithreading is not supported for now.\n");
+		return (ERR_FATAL | ERR_ALERT);
+	}
+
 	if (!LIST_ISEMPTY(&global_51degrees.property_names)) {
 		i = 0;
 		list_for_each_entry(name, &global_51degrees.property_names, list)