1. Implement PREQUEST message which passes a bit map of desired blocks.
   We still use the REQUEST message (start block + number of blocks) for
   full chunk requests as that is more efficient.  This message also
   includes a flag indicating whether it is a retry of a request we
   originally made or not.  This gives the server more accurate loss info.

2. More stats and tracing goo.


Frisbee client:

1. Add 'C' and 'W' command line options to specify amount of memory
   for chunk buffers (network buffering) and for write buffers (disk
   buffering).  The Emulab frisbee startup script uses these to partition
   up all the available memory on a machine.  Previously we were just
   using a fixed ~128MB even though our machines have 256 or 512MB of
   memory.  Also add the 'M' option which specifies the overall memory,
   internally dividing it up between chunk buffers and write buffers.

2. Add 'S' command line option to explicitly specify the server.  This
   allows us to make a feeb......