From: Manfred Spraul <manfred@colorfullife.com>

de_thread is called by exec to kill all threads in the thread group except
the threads required for exec.

The waiting is implemented by waiting for a wakeup from __exit_signal: If
the reference count is less or equal to 2, then the waiter is woken up.  If
exec is called by a non-leader thread, then two threads are required for
exec.

But if a thread group leader calls exec, then only one thread is required
for exec.  Thus the hardcoded "2" leads to a superfluous wakeup.  The patch
fixes that by adding a "notify_count" field to the signal structure.