SINGLEBUFFER, Generic Driver

[Hal Murray]

I just pushed the SINGLEBUFFER cleanup.

Does anybody use the Generic refclock driver?  If so, please test.  I made a 
small change which looks good, but I'm not setup to test it.

Eric, an item for your todo list:
  The serial input path for refclocks is two steps: input and receive.  I 
don't really understand it, but I haven't tried.  I'm pretty sure we can clean 
it up and simplify things.

It currently uses recv buffers which makes sense for the old way of grabbing 
stuff from a signal handler.  It may hang on to one if it gets a partial 
message.

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