Two changes to improve inter-thread message-transmission performance:

Two changes to improve inter-thread message-transmission performance:

1. Eliminate "msg_list_t" type and implement a fifo abstraction that
   is powerful enough to support the previous applications of message
   lists.  This eliminates the calls to malloc and free that were
   needed to build the lists, and replaces them by an aggregate,
   amortized cost.

2. When transmitting a message for which a receiving thread is
   waiting, call DosSleep to give up the rest of the transmitting
   thread's time slice, so that the receiving thread will run as soon
   as possible.