MINOR: threads: set the group ID and its bit in the thread group

This will ease the reporting of the current thread group ID when coming
from the thread itself, especially since it returns the visible ID,
starting at 1.
diff --git a/src/thread.c b/src/thread.c
index 2a7d3af..0b979f9 100644
--- a/src/thread.c
+++ b/src/thread.c
@@ -1019,6 +1019,7 @@
 	for (g = 0; g < global.nbtgroups; g++) {
 		if (!ha_tgroup_info[g].count)
 			ug++;
+		ha_tgroup_info[g].tgid = g + 1;
 	}
 
 	if (ug > ut) {