wait_on_damonized-start: wait only on the pid we want

Otherwise we can reap another thread's forked pid.

Changelog: Per Dwight's suggestion, use pid_t for argument.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: S.Çağlar Onur <caglar@10ur.org>