wanted to wait for a specific child; it always assumed you had specified WNOHANG.
This fixes the bug the fork_3-1 and fork_4-1 test applications reported. 3-1 still
sometimes fails, but that's a different problem (to be solved later).
* Also, it could return B_BAD_THREAD_ID instead of the expected ECHILD (for waitpid()).
* There was a race condition between testing for a thread, and checking its death
entry.
* wait_for_child() can now be interrupted in case it has to wait.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@18528 a95241bf-73f2-0310-859d-f6bbb57e9c96