Adding support for setting thread stop state when a process stops.

This re-submission of this patch fixes a problem where the code sometimes caused a deadlock.  The Process::SetPrivateState method was locking the Process::m_private_state variable and then later calling ThreadList::DidStop, which locks the ThreadList mutex.  Other methods in ThreadList which were being called from other threads lock the ThreadList mutex and then call Process::GetPrivateState which locks the Process::m_private_state mutex.  To avoid deadlocks, Process::SetPrivateState now locks the ThreadList mutex before locking the Process::m_private_state mutex.