Fix serious race in attach to many-threaded process

- the gist is that tasks continue running while we attach, so more tasks
  come into existence, or the ones that we didn't attach to yet disappear,
  etc.
- besides, we really can't enable breakpoints before we are done attaching,
  otherwise the still-running tasks risk running into them and dying of
  SIGTRAP.
diff --git a/sysdeps/linux-gnu/proc.c b/sysdeps/linux-gnu/proc.c
index a3fb3af..e3b71e5 100644
--- a/sysdeps/linux-gnu/proc.c
+++ b/sysdeps/linux-gnu/proc.c
@@ -102,7 +102,7 @@
 pid_t
 process_leader(pid_t pid)
 {
-	pid_t tgid = pid;
+	pid_t tgid = 0;
 	FILE * file = open_status_file(pid);
 	if (file != NULL) {
 		each_line_starting(file, "Tgid:\t", &process_leader_cb, &tgid);